What stops people is typically the USD transfer problem. MtGox and BTC-e are impossible to get USD out. Bitstamp and Kraken seem to work for USD. Kraken doesn't have much liquidity yet but they will as soon as they accept more USA customers.

Be really careful handing your money over to Bulgaria or Slovenia. Bitstamp (Slovenia) has been reliable so far. I think some of the negative stories about Bitstamp are planted by BTC-e shills. Really think BTC-e is run by wannabe Russian mafia.