guild-wars-2
Note: there's a good question on combat already, but it focuses mainly on using your own skills in combat, not on team play.
So far in GW2 I've done group events, and the main "strategy" I've seen is just zerging the opponents. In most MMO's I've played there's the traditional aggro + healer/tank/dps setup for dungeons and instances. I know this isn't the case in GW2, so here goes:
How does team play in instances work? What are the basics to know before hopping in with a PuG?
Overflow servers are created when the zone or world you're attempting to join is full. They're basically just another zone instance that's made. When you try to join a world/zone that's full, you will automatically be placed in an overflow server. You can earn XP and other rewards as normal on an overflow server, and it will consider you "in queue" for the "main" zone and once space opens up, it'll ask you if you want to join the main zone.
You will receive 3 options when the main server is ready to accept you:
Additionally, if you let the timer on the popup run out before you make a choice, it will default to "Reenter queue", leaving you on the overflow server, which will give you a chance to leave later.
Level downscaling will downlevel your character to the maximum allowable content level for that area.
So this will affect all your primary stats, Toughness, Vitality, Power and Precision. When you open your hero panel you'll see your main stats on the right in white, and next to them on the left you'll see your 'effective' stats in green. These are the numbers that you're going to be using. As these numbers are changed this will effect your damage, hp, crit chance, etc.
The thing with you feeling more powerful is that you still have the higher level gear. So for example, if you are lv30 you might have an amulet with +11 power. You go to a lv10 area, and fight with players who at lv10 can't even equip an amulet. Now I'm assuming, but let's say that the downscaling is a %, then your overall stats will be higher, meaning you'll be stronger at lower level because of your better gear.
You are by no means at all invincible, but you'll certainly fare a little better due to having more or better gear. The real killer is that as a higher level you will have traits and skills available which lower levels will not. You can also use your Elite skill still, if you've unlocked it. So there are some advantages to revisiting the lower level areas.
I think most people's experiences of this will be doing each of the race's starter areas once they've finished their own.
You can read more on the wiki, http://wiki.guildwars2.com/wiki/Dynamic_level_adjustment
Character level adjustments have a limit, however. The level-adjustment system won’t let a level 1 character go head-to-head against a level 80 character with much chance of victory—a level 80 character is still going to have more bonuses on their gear and access to a full complement of utility and elite skills that players just don’t have at lower levels. This is not to say that the high-level character will one-shot the level 1 player, but they will have a clear advantage.
http://www.arena.net/blog/mike-ferguson-on-guild-wars-2-world-vs-world
Edit
Since Jan 28th 2013, the downlevelling has changed. https://www.guildwars2.com/en-gb/news/dynamic-leveling-adjustment/
Best Answer
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
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!
Trash mobs
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.
Boss fights
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! :)!