Rick Stump over at Don’t Split the Party has an excellent article:

Good Isn’t Stupid, or weak, or nice.

Paladins don’t have to be simple, weak minded, naive fools. They can have depth and edges to them that makes them both interesting and far from an easy kill.

They should be a threat to evil and a threat to anyone who stands in their way.

Just as the evil villain is a threat to the forces of good.

Any DM who allows paladins, and any player who has, is, or wants to play a paladin should read this.