PvE at a Glance
I came up with the categories based on my experience with them and how I compare their differences. Due to people not liking the point values assigned I've moved this to just a three category system (best/middle/worst) to give a better overall idea.
Guardian/Juggernaut Shadow/Assassin Trooper/Bounty Hunter
Role Big Baddie Beater Sneaky Survivor The Collector
Passive Mitigation Middle Worst Best
Active Mitigation Middle Best Worst
Damage Smoothing Worst Middle Best
DPS Ability Worst Best Middle
Mobility Worst Middle Best
Control Best Middle Worst
Utility Best Middle Worst
Positioning Worst Best Middle
Single Target Threat Best Middle Worst
AOE Threat Worst Best Middle
Snap Threat Best Middle Worst
Difficulty to Play Highest Easy Moderate
(I'd love to make the chart more readable, it looks fine in the editor - for a tabled format look here: http://www.tankingtor.com/2012/06/ov...html#pveglance
Role Breakdown -
Guardians are the best at Boss tanking, this is still true even with the damage drops from 1.3.
Taking a Shadow is sometimes like having a DPS that can CC and having a tank all in one.
Troopers excel at rounding up trash mobs for AOE damage. They are also ideal for adds during boss fights.
Keep in mind the names are just for kicks, all ACs are capable of performing in all situations adequately.
Mitigation & Smoothing -
Shadows have by far the best defensive cooldowns and they're both on a short cycle so can be used (and should be used) frequently. They also self heal as part of their regular rotation. This is followed closely by Guardians and Vanguards lag behind with only a single effective defensive cooldown. The accuracy debuff can be situationally useful but usually when you need it, it won't work.
Coversely Vanguards have the best passive mitigation, including armor, shields and defense. They have the most flat DR (Damage Reduction) talents out of all the ACs. Guardians lag behind only slightly, and as of 1.3 Shadows fall far back down behind the pack.
If you take the two numbers together Guardians (15) Shadows (16) Vanguards (15) all three specs are quite close in effectiveness. One weakness is made up for by another strength.
This leads down to the damage smoothing, which is one of the most important aspects of being a tank. How easy are you to heal? Guardians are the clear loser here due to their lack of Shielding talents and reliance on armor and defense. Both Shadows and Vanguards have a talent to actively boost their shield rate and further talents to make it more effective/useful in their rotations. All tanks can work on these stats with Augments and itemization, but in general this is how it plays out.
Don't be mislead by the damage smoothing though, Shadows in general take more overall damage than either other class (see passive mitigation).
DPS Output -
It might not be fair to compare DPS on tank classes since none of them are designed to do damage, but it does matter (to a lesser degree).
After the last patch (1.3) Guardians have fallen quite a bit in terms of damage output. Their hardest hitting defensive talent was gutted and turned into a unique AOE instead. I can clearly see an effort was made to increase their AOE threat levels - but it did cost them in overal single target damage.
Vanguards also lost some of their damage with the dump of the surge increase on stockstrike. Since they didn't rely completely on that for damage it didn't hurt them as much, but I have noticed a drop in damage from them as well. Prior to the last patch I would have put them even with Shadows.
Shadows are constantly complained about for doing too much damage (in WZs) while in a tank stance. I believe this stems from them doing mostly force damage and relying very little on "white" (blockable) damage. Their main staples are also AOE attacks which probably helps them along where overall damage is considered.
Mobility, Control, Utility -
Vanguards have the edge in mobility with their ranged attacks. There are times when you can run through a few cooldowns and stay at the 30m range to do damage and still maintain threat. They lack in overall utility and control though - no knockback, one stun, their snare is talented but is tied to the regular attack rather than being on demand. They also have no "execute" skill to up their DPs at the crunch time of a fight.
Shadows also have a few ranged attacks, but their main one is a channel and can't be used on the run. Their speed boost is a great mobility helper, but it doesn't keep up with the leaps from the other two tanks. They do rank reasonably well for utility with a stun, kick, knockback and stealth.
Guardians have the worst mobility but the best control. Their snare is an AOE and can easily keep mobs close and grouped up. They have 2 stuns and an AOE Mez. They also have a push and AOE accuracy debuffs.
You can see how mobility and control was kind of balanced. Those will less control have more mobility and vice versa.
One of the things that I am most focused on when tanking is positioning. Where is the best place to bring the mob I'm fighting. how does that affect the rest of the encounter. Do I have to watch for knockbacks, stuns, AOE effects, cleaves. These are all things that a tank has to worry about.
Vanguards and Shadows have a huge edge here with their Pulls. They also both have ranged attacks that can be used to maintain aggro in a situation where moving in to close is a bad thing. The shadow has the slight edge in this category because he has both a knokcback and a pull. The knockback isn't perfect, but when you're in a range heavy group of mobs that you want to AOE, any little bit helps.
One of the major changes they made in 1.3 was to threat. All tanks recieved a 50% increase in threat generation. That is HUGE. Anyone complaining about threat after this change is just bad. I didn't have problems before the change, now, it's laughable.
With that said, there are still differences. Guardians have the most (and most effective) High Threat single target attacks. They are also able to front load a ton of damage to begin a fight which gives them a large boost in Snap Threat. They sacrifice AOE threat though with only one major move that does good damage and another that uses up too much resource to use more than twice or three times in succession without completely starving the guardian for the next short while. Holding AOE threat as a Guardian is difficult and generally they have to rely on their AOE taunt after spamming as much damage as possible.
Shadows got a huge boon in the last patch by making slow time "smart" so that it doesn't break CC. Prior to that holding single target threat on trash pulls could be difficult with poor positioning. I know I broke a lot of CCs trying to get the right seperation in so I could hold threat. Coversely there is no double that Shadows have the best AOE threat. Their main attack is an almost spammable AOE with high threat built in. With patch 1.3 their other AOE now also causes high threat.
Vanguards are really lacking on single target and snap threat. They really have to manage their resource, and while they can frontload a good bit of damage they run the risk of starving themselves too quickly if it isn't properly managed. They also only have a single high threat attack on a long cooldown that relies on a proc from their stance before it becomes active - so can't be used first thing in a fight. They do give Shadows a run for their money on AOE threat though - and are by far the highest in AOE damage potential.