One issue I remember with 3/3.5 was spell casters, spell penetration, and saving throws. Low level spells became useless at higher levels. Sleep would be useful for a few levels, but then quickly started to become less than useful because monster saves would sky rocket. Unfortunately, it's been a while since I played 3/3.5 and I no longer have any books (lost in a move), so I could be off the mark here. But, has PF changed any of this?