This has probably been talked about but i couldnt find any alive/not trolled thread about it so here it goes...
I do know most ppl are gonna say "we need to have roles, there has been some terrible mistakes with other games that tried something else" and I think they re right, BUT, I do think aswell that roles should matter for BOSS FIGHTS ONLY. What do I mean? Id like for dungeons/raids so called "trash" to not be trash but epic battles between armies/groups (ppl picking some1 and going 1on1, like actual old school wars), not a spamfest around a tank "picking adds" (In case you havent noticed, im pointing out things that discourage me from other games, like "trash" and "picking"). I just dont feel it would be fun at all for, for eg, DPS to keep smashing at mobs' backs while the tank keeps aggro, nor it makes any sense either. It does make sense on bosses though (they have to be attacking some1) and it is still fun because the fight itself makes it that you need to do some stuff apart from dpsing/healing/tanking. So, to sum up, I really hope they dont make "trash" the usual spamfest where dpsers/healers get 1 shot if they try to have any fun at all and tanks have to constantly "pick up" stuff. I mean, i love the real time blocking/parrying from skyrim and I do know they are implementing it, and its gonna be a sad story if its only useful for tanks aside from pvp. Thats all.

ZO has claimed that when entering an instance/dungeon you can expect the mobs to perform some of the same actions that you could expect actual people to perform in PVP, for example using synergies to take you restrict, kill, or lock down one of your group members like you plan to do to them. It seems like its gonna be a really fun strategic experience rather than just a tank and spank. They also said that these mobs and their class roles (mage, archer, warrior, etc) will be randomized upon entering the dungeon, making each time you enter the "same dungeon" you are getting different unique experience with different mob groups each time.