Feat or skill limitations based upon needing to fufill entry requirements into a PrC (Skill Focus [underwater basketweaving]! ...Yay?.) A full-level single-classed spellcaster can take whatever feats he wants. And if a PrC is so powerful that it's a must-take, then I certainly don't want it in my game. I'm looking at you, Radiant Servant of Pelor. The cleric in question was a cleric 5/RSoP 5 when I brought it to my DM's attention that the PrC was stupid broken, so he agreed to allow me to retool the character as a simple cleric 10. The DM in me can't abide over-the-top brokenness.