I've read from a bunch of interviews and sites that bungie decided that the highest level you'll achieve in the game is 20 because they want the players to focus more on finding new and better loot over time, and also to be lower so your friends have a smaller amount of time to catch up to you. Now that is either really stupid, or downright deceptive. I'm pretty sure that is just an excuse to charge you to raise the level cap with DLCs. People will get bored to death 1 week after release.