Guild Wars 2 is extremely unique - Being a lvl 65 elementalist, and having played all BWE's except for BWE1, I hope I can be of help answering your question.
Basic knowledge about GW2 instances
First off, the instances of GW2 are extremely hard for most people.
If you do not know the tactics of the bosses, you will die.. over and over again, I can tell you this from my own experience.
There is no way of somehow overleveling either, since you get downleveled. This way you must have a tactic approaching each boss.
Also, each instance has 2 modes, Story mode - Which basically involves a story which is somewhat related to the main storyline, and usually provides you with some lore on the game. This is always 'easier' then the other mode: Explorable mode - Note that explorable mode requires you to complete story mode first. Also, the minimum level requirement will be higher then story mode. In explorable mode, instead of following one specific story your party will be allowed to pick one of several paths (I believe 4 paths in each instance are available right now). These paths hold different stories, bosses and difficulty. Basically turning one instance into multiple different ones!
Conditions are also a very vital part of instances. Not only having conditions on your enemies helps. A very vital part is removing poisons, cripples etc. from yourself and your ALLIES. Using AoE condition cleansing seriously helps your team and will cause you to have a huge advantange over your enemies.
Basics of Combo Fields
Combo fields offer a lot of amazing bonusses and are usually very important in instances. For example: an elementalist might put down a combo field which is water. ANd if you then have a combo finisher - you get an AoE heal! Also, firing projectiles through fire fields deals more damage and burns.. and much more. To check all the combo field bonusses click this link.
Just check which of you abilities are combo fields/finishers and which are projectiles and use this to your advantage!
Resurrecting downed allies
Obviously, you want more firepower, more heals.. etc - So help up your allies that are downed, although do not risk yourself getting killed to much!
Mainly, the 'trash' mobs (mobs in between bosses) are already harder then in most other MMOs. These require target calling (press crtl+T). And the whole group should focus on this target. Note here once again - If you call the wrong target, the group might get completely devestated (as an example, picking to target a warrior first instead of a elementalist will usually not end well).
Obviously, you want to fight as less enemies as possible at a time, so stay aware of patrols.
Traps traps, and death.
Traps are also a major part of the GW2 instances, these are commonly found in many dungeons. They vary from mortar enemies on ledges, to basic spike traps to heads who fire fireballs. Most traps can be destroyed, or turned off. Look for mechanisms to turn these off (usually after a set of traps there is some sort of a lever, chain or something similar)! These traps can sometimes be at the same time as trash mobs, making it very challenging.
This is where I feel GW2 really shines. As an example, I will take one of the bosses in Ascalon Catatombs (AC), this boss will have two elementalist. These deal major AoE damage, so obviously - these must be killed first, while having your group spread out.
The boss itself, has quite a lot of health, and will deal quite some damage. Basically you have to kite him around as much as possible and try to dodge some of his basic attacks. Occassionally his weapon will glow and he will charge up a ability. He will then fire veins to all players near him, which you MUST. I repeat myself, MUST dodge. These pull you towards him into his next ability - a whirlwind. The veins alone already deal an incredible amount of damage, and his whirlwind deals even more damage. If unprepared, this one ability WILL wipe your complete party. However, if you all manage to evade the veins, you basically get him to do his whirlwind, and you can freely do damage!
Obviously, these tactics are different for each boss - note that most of the time, you get subtile hints from the NPCs who travel with your group, and these should definitly be followed! They usually help a lot. Try looking up the tactics for the next boss if you are unsure (note that right now, the game is just out and the wiki might not have proper tactics yet.. you probably got to figure them out yourself!).
As final note, I believe each boss will get "Defiant" stacks. These make the boss more powerfull, and these stacks can be removed with CC abilities. Make sure to use these and not let the stacks get to high, or it will get extremely hard!
I hope this is enough information - and if you wish to know more, feel free to leave a comment! :)!