Any melee group will work. Well, i'm still not sure on the warrior prot speced.

Reason Dk's work (and are going to get nerfed, but I will still be able to do heroics and anything else I want to do.) Is because of the fact that you have 5 tanks, 5dps, 5healers (sorta)...
Any group with ONE dedicated healer can do anything (as long as it's an AOE healer), single healers work, but when it comes to heavy melee, like you are talking about, I love Shamans over anyone. Totem make my world, and chain heal is great.

I had a Warrior/Rogue/Paladin group going, and I have 2 hunters that I might throw in the group to start doing dungeons in BC. Is it going to work? Yes, of course its going to. Why? Because you have a tank,3 dps and a healer, just like any other group match up...

5 Druids, 3 dps, 1 tank, 1 healer.
4 DK's and a healer.
4 warrior's and a healer.
1 tank 3 rogues and a healer.
4 Rogues and a healer (Really high avoidance for tanking)

Seeing the pattern? Having a healer will make any group work as long as you spec right and do the correct macros for all the right situations...
How am I able to do Heroics with NO healer but all DK's? Macros, and my spec. Are DK's OP? I guess, but so is a Rogue that knows how to play his class, or just about any other class that knows how to play.
You ever have a Paladin run up and OMFGONESHOTWIN!@ Yes, has happened on my DK's a lot. It's not hard for them to do that.
I've also been shut down by a couple of Hemo Rogues. I call bullshit. But guess what? I'm a DK, I'm OP...

Can any class run into any situtaion and do PVP/PVE?
No, not 100%, but guess what, they can change their gear out just as easy as I can and do another roll.

Can a Full BLOOD tank as good as a FULL FROST, or FULL UNHOLY?
Nope, he can't.

Can a Warrior be Prot speced and then switch his gear out to be Fury?
Nope, not really, but guess what, he dosen't want to be DPS, he wants to tank.

Druid can do the same thing, will they be 100% as another druid speced and geared for the job?
Nope, but they can still do it.

But hey, guess what, Dual-spec is coming out, and that means there will be no more QQ that DK's are OP because that means anyone can swtich a spec and fill another roll just as easy as DK's can.