Teamwork?
I agree with ExploderWizard, in part. One of the major failings of every edition is its forcing players to play particular roles to make the party successful. Clerics have long been indispensible, or at least, healing has - and I don't think that one of the possible 3E solutions, using a wand of cure light wounds, is really good for the game at all. In earlier editions, wizards were so pathetically weak and fragile that there needed to be a defender of sorts to guard that character.
4E's attempt to fix this, I think, went the wrong way. It defined classes by role, and then gave certain kinds of ability to classes within that role, which seemed to make it so that a variety of roles was necessary. But the designers botched this design by making four roles in an expected party of five characters, which meant that at least one role would be doubled. It is really no different than previous editions that way, except it is even more restricting - by playing the defender, one was obligated to play a particular way for the benefit of the party. Worse, two of these roles didn't really even do anything - I am hard pressed to know what it even means to be a striker (deal more damage? doesn't everyone want to do that) or a controller (forced movement?).
The worst result of this is that some of the best strategies in 4E involve using several characters of the same role - a party of clerics, or paladins, or warlords is fearsome because of the way the rules work. The system incentivizes the character choices it was meant to prevent.
I think a good way to proceed, in the basic version of the game, is to have characters be fairly self-sufficient. Not so that every character is an island unto himself, but characters don't absolutely require other team members - that way smaller than expected parties of adventurers can be accommodated by the rules.
Then, more complicated modules of the game could emphasize teamwork, through rules, but not by restricting certain kinds of actions to roles; instead, there could be benefits to having a variety of team members in an adventuring party. You don't need a cleric, but if you have a cleric and a wizard, there are options that open up that wouldn't have existed for the cleric or wizard alone. You could have things like the 4E warlord's benefits for allies, but you could type each effect, such that it would be a benefit to the party to have a bard and a warlord, say, but not as helpful to have two bards.
Rules ultimately drive player choices, because success and failure, and thus a large part of the fun, are a result of the way the rules work. If D&D Next is an edition that should emphasize teamwork, then it should do so through rules that promote teamwork, rather than punish its lack.
I'm not even certain that the game should be any other way. D&D has always been a team game - pretending that it isn't is part of why people want to play characters that are good for a story - the lone wolf / renaissance man - but not at all fun in a game where people get together to face challenges together. People have always railed at "needing" any given class - needing a cleric is not fun - but if having a cleric was guaranteed to make the other characters better, and that mechanical betterness was a result of high variety, then that is exactly what people would do.
I'm not sure that variety is necessarily the way to go - a team of rogues, or a party of "holy rollers" as my group used to play in 2E, can be a lot of fun too. I also think it is really important, as mentioned in above posts, to make the smaller party workable. But accommodations are more easily made once the design decisions are made for the typical group of adventurers. Making a varied party is as simple as making the varied party mechanically better than the homogeneous one.
