Just to confuse your further. Our group has a tactical warlord, who's healing is surpisingly good and has all sorts of ways of granting extra attacks and bonus's to hit and damage to allies. Invaluable contributer.
Question to OP. Do you have a group of "glory hound" players. Normally that is the case when everyone wants to be a striker. Before starting, encourage them to broaden their thinking 4e is VERY different to previous versions, and non striker classes are far more interesting than they were in the past. You really do measure contribution in this game by more than damage alone, in fact, they are going to have a tough time if all they do is damage...
I have a player who used to be a "striker only" mentality. He agreed to "tank" the group (dwarf fighter). To his surprise he gets a real kick out of locking enemies down, frustrating enemies and drawing fire unto himself. Another such player went a wizard, and he just loves causing chaos amongst enemy ranks. These guys literally give audable yells of "YES!" and raise there fists in the air when they succeed.
Pre 4e rewarded damage output, 4e rewards so much more. Help your players understand this before you proceed, or you wont get the most out of it (and are probably better off with 3.5 or pathfinder)