The only serious concern I've found with Bitcoin so far is the probability that Satoshi has one or two million stashed away somewhere. That would represent up to 33% of current bitcoins and 10% of eventual bitcoins.
http://forum.bitcoin.org/index.php?topic=9745.20I can immediately think of two implications:
1. Bitcoin is vulnerable to massive market manipulation in the future by a single individual. This will deter anyone wanting to use a currency that floats freely in a market that's both unregulated and unmanipulated.
2. This could eventually represent an enormous amount of wealth. If the price rose to $10,000/BTC that's about the $20B that Larry Page is worth, so no problem. But if Bitcoin rises to be a significant fraction of the world's economy, that's a whole load of power in a single individual's hands. I hope Satoshi doesn't turn out to be The Joker.