I mean how will this be any different from TT where different champions are at different levels of balance? Actually I see this as being even more of a difference in balance since it is also a different game type.

A) Dominion is 5v5
B) assuming you meant SR, different items will be available in Dominion
C) there's a contant aura in effect in Dominion, which helps to balance.
D) even if champions aren't in the same "tier" between game modes, they will all be VIABLE.

Mundo isn't balanced for 5v5 he was nerfed because he was a pubstomer in 3v3. Even tho riot "says" it'll balance around items..i get the eerie feeling that they're going to start "tweaking" champions after dominion comes out.

rage hard.

of course not all characters are going to be viable in every game mode. entire classes of champs are not viable in 3v3s, simple by the nature of the class. Anyone who says that Soraka should be buffed because she's not a viable champ in 3v3s same with most squishies.
Dominion will make certain characters shine, but at the end of the day don't rage. there are dozens of champs to chose from.