I'm with the guys who say security for the average Joe is the biggest obstacle.  I work with many business people on a day to day basis and most of them have no clue about even the simplest of security measures and what the consequences are if their data is stolen.  With bitcoin, we are talking about securing wealth.  People MUST understand it.  They would not understand that no one should ever see that private key printed on their paper wallet.  No camera of any type should come near your paper wallet.  Trojans, man-in-the-middle, etc are all potential threats that could potentially wipe out a life's savings in a heartbeat with no recourse.  It is for these reason i have yet to encourage anyone to try it.  Let the pros work out the kinks first.  I believe a viable solution will come but until then bitcoin is not ready for mainstream.