I've played a lot of campaigns where the party would end up getting stuck somewhere without the possibility of resting to regain spells.
So frequently the wizard would run out of spells first. The fighter and thief would still be able to use all their abilities. The cleric would get to pick and choose who got healed and who didn't -- thus the cleric has power over the others.
In 3.0/3.5, clerics don't have to be responsible to a deity, so that's a big slice of power right there.
If your DM allows clerics to take the Domain of Madness, they can jack up their extra spells in proportion to their levels. That means the cleric will have enough extra spells to cast spells alongside the wizard and still have spell slots left over with which to heal everyone.
Oh, yeah, and they get heavy armor and maces, so they get a chance to at least stand at the front and try to hit something while the wizard is cowering at the back of the party and feeling bored and worthless.
Even if they never run into undead, clerics are a big win. They are the class most able to defeat an entire dungeon solo. Everybody needs clerics, and clerics don't need anybody. This has been altered a bit in 3.0/3.5 with wands of healing, but wands can get broken or used up.
Recently, however, the Complete Arcane introduced warlocks. Warlocks are something antithetical to the old spirit of the AD&D magic-user: they are magic-users who never run out of offensive ranged attacks. It's a great idea -- Superman never ran out of heat vision -- and it's very powerful in campaigns where the DM likes to isolate the party in hostile territory and wear down their resources.
To give you some idea of the old ways, here's a quote from Role Playing Mastery:
In both the D&D and AD&D games, the spell-using power of
PCs is controlled through the use of a system that requires study
and memorization of magic spells before they can be cast. Then,
once a spell is used, the ability to cast it is erased from the
character’s mind until that character again takes time to study and
memorize the particular spell. Well, some years back, there arose a
line of thinking that asserted that magic in a fantasy game was
best expressed in terms of spell points-characters should be able
to cast a certain spell often and repeatedly, with each usage simply
costing the caster a specified number of “points” from his magical
ability. The D&D and AD&D games were criticized harshly by
advocates of this approach for being behind the times. The fad
lasted for a time, with spell-casters spewing forth streams of
sorcerous stuff as if they were magical Gatling guns.
Everyone wanted to be a magic-user of that sort-but what could stand
before such a character? How much fun is a game in which any
challenge or problem can be overcome by calling up yet another
spell from a seemingly limitless storehouse of energy? Good-bye,
spell-point magic system. This is not a condemnation of the idea
of using a point system, but the point system as advocated did not
fit the D&D or AD&D game system spells, rules, assumptions, or
spirit. The idea is workable still, but needs its own body of
surrounding material to operate effectively.
Warlocks are even worse than the spell point system, because they never, ever run out. For many campaigns, that's even more broken than clerics.
However, what constitutes an overpowered class depends on the challenges the DM throws at you. If you're in a situation where everyone has to be stealthy and sneak up on enemies for a one-shot kill, thieves are overpowered.