I believe this because I hate tanking and have never liked playing melee classes (my main is an elemental shaman and my highest alt is a warlock) and yet I'm having HUGE fun in the beta realm playing a Death Knight. I've played the class PvE to level 65, trying all three specs multiple times. I no longer fear dueling. I solo group quests that are above my level. I can approach elites one level higher than myself, alone and with no fear. I just run into a group of mobs with little strategy and almost always survive.
Although I think the clarity of the purpose for each spec is still a bit muddy, I see Death Knights mostly filling a tanking role. (Perhaps this was Blizzard's response to the tank shortage? Make a tank so fun to play that lots of people will play them?) And yet Death Knights have soloing capabilities like warlocks and hunters -- only they survive better. They are by far the best soloing class I've tried. For illustration purposes, I even made a goofy movie showing how badly you can play a Death Knight and still take some serious names without dying once.
Blizzard has proved that it knows what it's doing over the last four years. However, the current class system even without Death Knights has always had complaints about balance. As even Blizzard itself has admitted, adding a new class is not easy. Perhaps I am wrong and the tweaks they are making to the existing classes for Wrath will make everyone feel like they are overpowered. As it is, though, playing my new Death Knight gives me such a feeling of power that I've never had in the game before that I can't imagine that there won't be a lot of other players like me when Wrath hits the shelves this fall. Will this be good for class balance? More importantly, will anybody care?