Rogue/Thief: The good role for the impulsive tactical thinker. Using their combat and dungeon abilities requires initiative and coordination, but the abilities are not depleted by use, so the big picture doesn't play a part. If you don't have this type of player: Just play instinctively and roll with the punches, stabs, crushes and KABOOMs. Or throw a chicken down the stairs in front of you.
Wizard/Magic-User: Knowing this role well requires a grasp of a part of the rules - spells - that other classes can just ignore. This is a role best suited for the "specialist" player who enjoys working with complex systems and bending the rules. The player has several big, one-shot effects at his or her disposal, and especially at early levels, having the strategic big picture and knowing when to use each one is crucial. Having a strategist's mind also overcomes the critique that the magic-user's player has nothing to do when the spells run out; this player is likely to take over managing the party's other resources. If you don't have this type of player: Invest in oil, acid, offensive scrolls and potions.
Cleric: The cleric plays a relatively simple role, like the fighter, but supportive rather than offensive. The times to intervene are clear: heal wounded party members; turn undead when they show up. This should appeal to the player who likes to support and hold the party together. If you don't have this type of player: Invest in protection scrolls and healing potions.
In D&D, though, the cleric class partakes of other player roles - an armored blocker like the fighter; a caster of offensive spells like the wizard. This dilutes the class somewhat and may be part of the complaints that it is too bland or overpowered.
One of the reasons I'm eagerly looking forward to further development of the "all cleric spells work like turning" idea on Nine and Thirty Kingdoms: it pushes the cleric to further differentiation from the magic-user. The cleric becomes a manager of uncertainty over time rather than a dispenser of sure-shot one-shots.