I've yet to get my team in place - really need to work on levelling my priest, however I am tanking heroics for quite melee heavy guild teams.

A dedicated tank, specced correctly using high threat abilities and given a lead time to build initial aggro (if need be) should be able to hold aggro from your DPS.
Mobs that aggro dump may cause issues, but you would just need to hold off on DPS till the tank reacquires it or aggro fade on your dps and taunt with your melee.

There is a trigger happy Ret pally who hits the mob the same time I do so pulls aggro, but I just tell him to back off and let me get a couple of swipes in first.

General concensus is that casters are easier, requiring less movement and placement, can be placed out of the aoe damage of some bosses etc.

I just want to prove that my team can be viable.