Radiant Servant of Pelor is a good example of a class that people yell "OMG broken" over that just isn't. You have to sink several ranks in Heal to qualify, and your domains get chosen for you. You also have a feat chosen for you. You do get martial weapon proficiency, some nice SLAs, and a few enhanced heals each day, but you do get a reduced hit die (d6). If I were writing it, I might make it cost a level of spellcasting, but I think that would unduly penalize multiclass characters entering the class. It's strong, I wouldn't necessarily allow it, but I don't see how you could weaken it without it becoming just a core cleric. It certainly will not, under any circumstances, break a campaign. Even clerics looking for a power boost would have to focus on specific strategy to find Radiant Servant broken. You could make the class freely available and you still wouldn't have a ton of them, maybe not even most clerics.
For broken:
Hulking Hurler (overburdened heave lends itself to incredible abuse with great ease; anyone can get dozens of dice of damage, liberal use of options can yield several million - yes, million - d6s)
War Hulk (cute idea, but in addition to having a nonsensical loss of skill ranks, +2 Str per level, every level, is both unbalanced and conceptually strange)
Chosen of Mystara (miracle 2/day with no XP cost at 17th level)
Ur-Priest (9th level spells with full caster level in your late teens; also, conceptually identical to an Evil cleric without a deity)
Frenzied Berseker (great power, with the drawback you will eventually destroy your allies and the campaign, unless you abuse the BoED to remove all the drawbacks)
True Necromancer (its combined caster level is usually curbed by a careful reading of the rules, but there are corner cases that can lead to caster levels in the high 30s before epic level)
Illithid Slayer (makes you immune to a great many psionics in a psionically oriented campaign)
Not broken:
Radiant Servant of Pelor (strong, but not overly strong)
Fochulan Lyricist (gives you everything but a d12 hit die, but the price of getting there is so high it's barely a consolation)
Mystic Theurge and its relatives (in general, high level spell access trumps low level list access)
Archmage (yes, it gives you nice benefits, but instead of getting extra feats, as a wizard, you spend them qualifying for this class)