Right now warriors are the weakest class, if you had 4 equally skilled players playing each of those classes, that is. DK's are largely OP right now. I feel like my lvl 65 DK has more survivability than my 400 ilevel warrior.
Oh we're entirely viable, its jsut that the warrior toolbox is the least suited to thetype of encounters that make up Dragonsoul whereas the DK's toolbox is rather well suited. It's also rather unfortunate that our DPS is generally well behind the other tanks, almost 10% behind DKs on all size/difficulties.
This is argueable, imho paladin are the weakest class by far. They can only reach ctc cap and start stacking stamina afterwards with no option to further reduce their incoming damage while us warrior can aim for critblock cap. Might depend on the raidsize but i prefer a warrior over our blocking brethren any day (please note that basically none of the fancy bubble/AD stuff works on heroic mode).
I guess they're a little more equal, Warr's having crit block and mobility, paladins have a wider range of CDs, etc. But DK's and Druids are leaps and bounds ahead of both in terms of survivability and DPS.