I think the answer to the question changes depending on whether or not the PCs are all the same build of the class. A party of all shielding clerics would be terrible, but a party with a mix of Str, Laser, and Shielding would be great.
A party of all one type of Warlord would probably not do well, as their init bonus and action point bonuses would not stack, but a party with one of each type would buff each other amazingly and have absurd action points. Also, the Warlord benefits from having other melee fighters, and so having everyone in the party be a melee fighter helps them out, although the lack of ranged attacks will be a problem.
I think an all wizard party could do very well, actually, particularly if they are all Staff (for good defenses) or Orb (as a good Wisdom, Thunderwave, and Cloud of Daggers adds up to a lot of damage as they ping pong enemies through the Cloud(s) over and over again). With good bursts and blasts, they have solid melee and ranged capabilities, and the lower single target damage should be made up for by constantly hitting multiple enemies (if the enemies don't cooperate, then one PC will just have to set them up for the others).
I think the all Warlock party can be great if they have a hexhammer or two and a mix of the other types, but an all Feylock party would be a contender for worst party.
I think an all Bard party would be fine, with plenty of buffs and debuffs, healing, and the ability to grab melee powers. In fact, if you assume that the party all starts out as the same build, they are one of the better options, as they can pick up both melee and ranged attacks regardless of build. They will also totally rock skill challenges.