In a two leader party [warlord and cleric] you can do some out of the ordinary stuff. Since you have two leaders, and the warlord is inspiring, you can probably afford to have the cleric be a battle cleric that goes the defender route. That means that the defender should likely be a fighter or swordmage [since you probably don't need extra healing from the Paladin]. If the cleric is battle, you'll probably want a ranged striker to give the party some ranged attack power in addition to the wizard. So, that would probably mean an archer ranger or warlock.
In general though, party make up will look at having the leader and defender roles locked up, having some ranged power in addition to the melee fighters, choosing the right skills for rituals and skill challenges, and making sure that power selection works well with other members of the party. Powers that give combat advantage are great when a rogue is in the party. Powers that gives a bonus to all attacks, instead of just one attack are great for helping out a ranger. It's also good to avoid overlapping effects that won't benefit each other ... an infernal warlock may not need other characters giving him temporary hitpoints, for example.