Iron Sky
Procedurally Generated
I really think it would be hard to play without a leader-type. Not impossible, but darn hard. We played our first game without a striker, and that went fairly well (though fights were long). I also think defenders and controllers are fairly optional (though quite helpful). But not having a leader sucks hard.
Second this ^^
I can't imagine trying to play without a leader, unless everyone had quick-draw and the DM gave out healing pots with every batch of treasure and every town had a potion store.
In our first playtest group we tried had one of each role, and we almost casually mopped our way through 3 1/2 levels of dungeon, regardless of what composition of enemies the DM threw at us. Never even had anyone drop to 0 hp and only once did anyone stay bloodied for more than a round. Was like a well-oiled machine. My favorite part about it, aside from being kick-ass, was that each person had a specific job that was different from everyone elses in significant ways:
Our Paladin was dedicated to getting in the enemies' faces as soon as possible and draw the attention of as many of them as he could, challenging the biggest of them.
Our rogue was dedicated to positioning himself where he could get CA and do as much damage to the biggest enemy he could find, as fast as he could.
Our wizard was focused on looking for how to get the most targets in his AoEs as he could, especially if they were minions.
Our cleric was always watching everyone's health, with healing anyone who was bloodied being his highest priority and distributing buffs being his secondary.
Everyone felt unique.
For our "real" group has no controller, no defender. One of each striker, one cleric. Damage gets spread pretty evenly amongst the ranger, rogue, and cleric (warlock is fey, so can usually teleport to safety or eyebite single targets that are focusing on him). At first, our three strikers felt very similar, especially since we all opened up with ranged attacks. Now at 6th level and we've each become more specialized there's enough diversity in our powers that we feel unique.
Fights can be "spikey" without a defender(anyone might go down if enough enemies focus fire and/or the DM rolls well) and also due to our extreme damage output (a few lucky rolls can drop half the enemies in a round or two, swinging the battle in our favor, while bad rolls can put us on the edge quickly). On the whole though, the group works great - only fights with x2 our "balanced" xp total tend to be at all difficult.
I think as long as you have a leader or two, decent tactics, and every player can take a hit or two before going down, it'll work fine. As long as everyone can adapt and select feats and powers that work to fill the parties' gaps, it should be fine. For example, our warlock selects AoE and controllerish powers whenever he can to help cover our lack of wizard and the rest of us have at least some focus on increasing hp/defenses to help cover our lack of defender.
If the players start without a focus on group balance, it'll probably be ok, but if they continue to disregard it, they'll eventually hit a fight where diversity would keep them alive and over-specialization means they fail. I think that playing well tactically together and making smart personal choices far outweighs having group balance as far as survivability goes. A well-balanced group that has people not playing their roles well will probably do far worse than an "unbalanced" group that use good tactics and makes synergistic power choices (like our current one).