Just levelled my mixed team to 60:

1 x Protection Warrior
1 x Elemental Shamen
1 x Arcane Mage
1 x Holy Priest

All from patch 4.0.1. Been an incredible learning experience and great fun, gradually I have incorporated ISBoxer clickbars for each class so they can execute their 'specials' as well as fairly complex macros. Levelling I sometimes used dungeon finder and therefore got an 'outsider' into my team, it has been really satisfying to know I am tanking, healing and beating the extra player on the DPS table! This is not because I am in anyway uber, it's all down to the power of ISBoxer.

Dungeon's are ideal for mixed teams, in fact I have found it easier than tanking in a normal group.. because at least the DPS is all on the right target all the time.