As it is right now, everyone play dps, and healers / tanks arent really needed, and it isn't really worth playing tank/healer because of low healing or low defensive - making raiding/dungeons very boring.
My suggestion is:
1. Remove possibility to revive in combat while inside dungeon/raid - This way healers are needed to raid/dungeon.
2. Nerf stamina for DPS items by 20-40% eventually compensate with extra damage stats like agility, intelligence, strength, etc. - This way tanks are needed to raid/dungeon and DPS is needed to keep moving.
3. Up defensive stats for tank items, (Armor, Stamina, Defense rating, dodge, parry, block, block rating.) - Right now there's not much difference on the stats if you're DPS geared rather than tank geared, tanks have a little more hp, but die just as fast as pure dps's.
4. Buff healers in general so they can keep teammates alive, right now they cant outheal most dps from raids like zul aman.
Basically, better defensive for tanks, less defensives for dps, proper healing for healers.
Right now every single dungeon/raid is soloable with decent gear, bring teamplay into the game and the population will go up! :]