I hit level 20 earlier tonight (21 now thanks to some fancy gear). I've played a lot! I also knew exactly what I was doing in Old Russia, since I've done it all 5 or six times before. Went a lot more cautiosly on the moon, and I haven't done much of Venus at all yet. I also made sure to always be clearing bounties, and I played a good amount of PVP in the crucible, which awards bonus XP in a certain game mode (rotates), and gives lots of XP anyway if you run good at killing folks