This is one of those old rules that got deleted with the old forums and hasn't come up enough to get re-posted. If you've made a character that bends one of these rules in the last few years, no biggie, you'll be grandfathered in. From now on though if you name a character in such a way that breaks RP immersion, you'll be asked to re-roll the character.
Examples of what not to do include (but aren't limited to):
1. No names in l33t. This should go without saying.
2. Please don't use prank names such as Seymour Butz, Oliver Clothesoff, etc.
3. No names from well known characters in popular media. No drow named Drizzt, no iron-clad fighters named Tony Stark, no goblins named Justin Beiber, etc.
4. No offensive names. Don't name your half-ogre "Big Mudderfukker".
5. Don't include a title in your character's name. No character will start out as a "Lord," "Lady," "Sir," "Professor," "Dr." etc. These sort of titles are rare and only given out by DMs for excellent long term RP and IC events, but will never appear in a floaty name due to engine limitations.
You can name a character after a god so long as your character never implies that they are, or are related to that god. "Odin Johnson" would be acceptable, "John Odinson" might be pushing it.
edit: added a couple I forgot