Too many times have I seen people make poorrly deisgned characters and become upset from it. This is for beginners making characters for the first time. A lot of it seems like common sense, I still see a lot of basic mistakes however. They seem to help out my PCs alot, but then again they usually make me make their characters. I don't like giving unsolicited advice to people but here it is regardlessly.
Rules by which I make character by.
Ahem, Smurf.
#1. Don't try and make your character do everything. The game is designed for characters to be dependent on other classes/characters.
#2. Don't spread your ability scores out too far. Having a +1 in everything is great ideally, but when you are a barbarian with a +1 con and str you sort of just boned yourself.
#3. Don't try and make your class something it is not. It rarely works.
#4. Make the most out of your ability scorres. Have a High Dex? Use light dex friendly armor, grab as many dex skills as possible.
#5. Fill a role the group needs and try not to copy another role.
Please keep adding to this list!