5. Clerics have to prove their worthiness to their deity before getting a spell at 2nd level
Honestly, this one is just a personal preference with very little rational to back it up. I think it stems from my preference for low magic games. In fact, most clergy in my campaigns are usually non-magic casting priests. It may also be due to my preference for sword & sorcery-esque games.
I read a little while ago on Grognardia about how James wishes he had combined clerics and magic-users into a white wizard/black wizard thing for his campaign which I think is an interesting idea for a S&S-type game.