I run only mixed groups and have been challenged by WotLK dungeons but have been able to do all the ones I've tried.

My groups have pallies or warriors to tank, priests or shaman to heal, and a mix of shamans, boomkins, hunters, and mages to DPS. None of my teams has more than one of any class (actually, one has two shaman - one resto and one elemental).

At level 72, three different mixed teams of mine did UK.

At level 74, two mixed teams did Nexus.

At level 75 I did Nerub with one team.

At level 75 I did Drak with one team.

At level 76 I did Gundrak with one team.

In every case I wiped about 3 times the first time I ran a new dungeon on the first team that did it.

Set up prep macros, blow cooldowns for offense macros, and "the *#&% has hit the fan" macros as well as basic DPS and healing ones. Read the comments on the bosses from Wowhead or another site and you should be fine. Not a cakewalk, but the fights have been the perfect difficulty from my point of view - challenging enough to be exciting when you win but not "beat your head against the wall" hard, either.