My innner concept of a 'balanced' party is one of similar level and with the basic abilities covered: healing, offensive magic, sneaking, toe-to-toe fighting.
This is not to say I particularly *like* that concept, but it pretty much remains one of the key assumptions of most adventure authors, and I can sympathise with that. It is also a concept that has infused almost all of my players, such that whenever we put together a new party for a campaign or one-off, it sounds more like a shopping list than an exercise in fun or imagination ("Tank? Check! Sneak Attack? Check! Magic Missile? Check!").
If you're going to break this convention then the DM has to accept that he's making work for himself, that he's going to have to tailor the encounters to the abilities of the party, or they're going to be in trouble. I spelled this out to my group when they started the latest Dragonlance campaign, and while there was palpable skepticism on the faces of at least two of them, they've actually tried something different for that game, and it's working out fine. Yes, it's slightly more work for me, but I get to watch them use more esoteric abilities, classes, and races, which makes the game much more fun for me (and really, that's all that matters

).
Levels are a different matter. Anything more than 1 or 2 level's divergence from the party average and those PC's are going to be in serious trouble. Yes, they will get a bigger reward for their trouble, but it's no fun for players who have to constantly come to their rescue for the privilege.