Try Before you Buy
Even more so than the other classes I have played, it took me quite some time to really get into the groove of my guardian. There are a number of reasons for this, and I'll get into those in a bit, but for me, it really underscores a notion that I have - and that is that you really shouldn't make a judgement on a class until you've given it an honest go. I'd say playing it at least until level 20. I know that for both my mesmer and my guardian, at level 20 I was still trying out different weapons, builds, and playstyles, before finding the right combination for me. And something that isn't talked about as much as it should be I think, but a class playstyle is not shaped by its weapon choices alone. It is truly shaped by the combination of your weapon choices and your utility and trait selections. It's not until you have 3 or 4 utility skills, and 10 or 15 trait points reinforcing those selections I think before you get a good feel of how a particular playstyle will feel.
So though I've said it before, it bears repeating again. Re-allocating trait points is easy and cheap - don't be afraid to do it. A lot of people advocate using the training ground in the sPvP lobby to play with builds, and I also think that is a fine idea. But for me personally, I can't get a feel for a playstyle against an attack dummy - I have to get out in the field and use it and use it. So if you're struggling with a class - visit your trainer - change things up. Find other builds online that sound like something you might enjoy, put them together, and then modify to taste - that is what I do. You might find a build and a playstyle for a class that you didn't even know existed - and that you enjoy!
Avoiding the Default Spec
I really think just about every class has sort of a default, or go-to way of playing. I don't have any data backing this up more than just my own anecdotal observations, but it certainly seems to be the case. For instance, I think for mesmer's in PvE it's greatsword. For elementalists it's staff. For thieves it's dual-daggers or sword/pistol, and for guardians, it appears to be greatsword. And this is nothing against those players that play with that playstyle (I myself play my thief almost exclusively dual-dagger), but I tend to generally look for playstyles that are a bit off of the mainstream. Also, I knew that I specifically did not want to build a DPS guardian - but rather one aimed more at a strong support role. I mention this so that you know where my choices below come from, and why I'm not your standard leap-and-whirl guardian (though I did give that a go for a bit, and it is lots of fun in its own right!)
You Are Not a God
So as I mentioned above, early on I struggled quite a bit with my guardian, and here's why. You see, what most people that don't play guardians don't know about guardians is that we actually have the lowest initial hit-point pool of all the classes. Yeah. We have a ton of ways of mitigating damage to make up for it, but once a mob or group of mobs gets past those defenses, you will go down, and go down quick. And early on, when you haven't got many of your traits, or you're struggling to figure out exactly how virtues work or you're just (as was the case for me) plain stupid and throw yourself into situations where you are faced with overwhelming odds, well you will die. I'd read so much about how godly guardians where, how they were these unstoppable defensive juggernauts, and I kept throwing myself at three and four enemies at a time or leaping into a pack of centaurs by myself, and 15 seconds later finding myself sucking air, and going Double You Tee Eff?? I never died this much as a mesmer! Well as a mesmer I was bit smarter about how I played as well. So just a word of caution if you're starting a new guardian, or thinking about playing one. Those defensive capabilities do kick in, and eventually you will find yourself easily defeating five or even six opponents at once, but only after you've earned some traits, and only after you've got a good feel for the class. But in the early stages - like before level 30 - take it easy.
Understanding the Weapons
Guardians have a lot of weapon choices. And the thing is, the effects of those choices are often not intuitive. For instance, you might have in your mind a mental image of the guardian as this plate armor wearing sword and shield wielding defensive tank, but in reality, when you start looking at the skills associated with the weapons, you might find that scepter and focus might be better for you, or sword and torch! Additionally, many of the weapon abilities have somewhat subtle, secondary effects, and understanding and utilizing those secondary effects can be key to your - and your group's - survival. This is especially true when it comes to combinations - but more on that in a bit. So what do I mean? Well here's a for instance:
Shield is, as you would expect, a defensive weapon. But it may not be defensive in the way that you expect. For instance, shield doesn't offer any block. What it does offer is a way to grant protection to allies, and a dome that pushes enemies away, blocks projectiles, and can be detonated to heal allies in it's confines. The focus, on the other hand, with Shield of Wrath, offers a fantastic triple block in what is arguably one of the best personal defensive abilities in the game. And it offers condition removal and regeneration for allies. The upshot of this is focus is actually a better personal defensive weapon, whereas shield is more of a group-oriented defensive weapon. Sword on the other hand, is primarily an offensive weapon. It offers a variety of offensive attacks and a gap closer, but almost no group utility at all. So if you pair sword with shield, you're pairing what is essentially a personal damage dealing weapon with a group-oriented defensive weapon. That's not to say it can't be done and you can't make good use of it, but you need to be aware of exactly how those things work to take best advantage of it. There are a number of weapon combinations that are like that. The important thing though is to drop any preconceptions you might have of how particular weapons should work, and instead learn how they do work, and then put that knowledge to best use.
Virtues are the guardian's signature ability - the thing we can do that's different than what everyone else can do. For whatever reason, I never felt like virtues where really well described in the tooltips, and I actually had to do quite a bit of reading to figure out exactly how they work. It's really quite simple - just not very well described. Here's the skinny: The stuff that's listed under the "Virtue" keyword in the tooltip you get for free, all the time. You never have to do a thing. The stuff that's listed under the "Activate" keyword is a group wide buff that applies to you and your nearby allies when you activate it. That giant glowing flames that appears around the virtue bars is nothing more than a vague and not-very-helpful indicator of when Virtue of Justice is going to go off again. The flames get larger as you get closer to it going off.
The biggest thing to remember about Virtues is to not forget about them. Those group buffs are significant - granting Aegis to everyone in your area when Kol Skullcrusher is about to slam his foot down can make the difference between wiping on him and defeating him. Every time someone starts to help you out in a fight, or a group forms adhoc around some event, start using those virtues. The second thing about the virtues is to realize that, combined with the Renewed Focus elite skill and the Virtues trait line, you can create an entire build around virtues that can provide an incredible amount of group utility. I'm playing around with just such a thing now.
I saved this section for last, because I feel it's far and away the most important. More than any other class I've played so far, an absolute key to playing a guardian well, and enjoying the guardian, is to understand how you can create combos, what those combos provide, and recognizing the synergy between the combo fields and finishers provided by your weapons. Centurian, over at Guild Wars 2 Insider, has written an excellent and exhaustive guide on the various combos guardians have available, and it is highly recommended reading. Of course I've known about the combo mechanic since BWE1, but I always thought of combos as mostly bubble shields you can get for a small temporary bonus. Guardian combos are a whole different thing. I started noticing when using my weapons I was often kicking of Area Retaliation. That made me aware of the combo, and once I became aware of it, I switched up my weapons and rotation in such a way as to take advantage of it as much as I could.
For instance, here is my current build, and how I'm using it. I typically try to start the fight with mace and shield current. I don't use any gap close, just swing the the mace to get them mob's attention. The first attack is usually blocked by the Aegis that is up from the virtue. Immediately hit Protector's Strike, to gain a block on the second attack, which is usually the mob's heavy hitting ability. This also sets the mob on fire due to Defender's Flame in the Valor line. Follow up with Symbol of Faith which paints a light field and grants regeneration. Immediately switch to hammer, and hit Mighty Blow, which creates the Area Retaliation blast finisher. Wait a few seconds, then hit Purging Flames from the utility line, and Mighty Blow again now for a fire blast combo finisher that gives myself and nearby allies three stacks of might. If it's up, I might also hit Ring of Warding, which also generates a light field (though it doesn't say so in the tooltip), hit Mighty Blow again, for another stack of retaliation. If not, wait for the third attack in the hammer attack chain which grants another light field, and again use Mighty Blow to finish the combo. Switch back to mace and shield, allow the auto-attacks to get some regeneration going, and again use Symbol of Faith. In the first 10 seconds of a typical fight, I've applied multiple stacks of regeneration, retaliation, and might to myself and my nearby allies. And that's in addition to whatever I fire off from virtues. Rinse and repeat as necessary, applying fields and finishing them, to continue to grant your allies a number of boons. Again, it can't be stressed enough, to really get the most out of your guardian, know and use your combos.
Sadly, due to the rather poor UI implementation of boons and conditions, many of your allies will never even be aware of how much you're doing. The good ones will. The good ones will realize what you're providing, and the really good ones will even take advantage of it - like firing projectiles through your light fields for AoE condition removal. But this isn't WoW, where people see big green numbers with your name attached to it to realize how much you're providing. Through the right combination of weapons, traits, and abilities, your guardian can provide a variety of roles, and it's one of the few classes that can truly excel in a support capacity. But just don't expect a lot of praise for it.
I've used my guardian in almost an exclusively PvE aspect. I hear they're great in sPvP at holding locations, and even useful in WvW. I've participated in all of the winter event dungeons - though admittedly those are hardly dungeons - and thus far done Caudecus Manor in story mode with a pickup group. I'm really starting to jam on the guardian now, and feel like I've found my footing, and am really enjoying the class. I feel that I truly make a difference when I show up to group events, and have "tanked" a number of champion mobs, using the variety of control abilities in my spec. I look forward to getting further down my trait lines, and doing more dungeons with her at 80. In fact, it's quite likely this may be my go-to gal for doing fractals, and starting to work on that Ascended gear. Thoughts - comments - your own favorite guardian build? All are welcome in the comments!