I think what holds many domainers back is 2-3 things.
1, Burden of Knowledge - i.e. We have a good sense of the digital economy bill, drs, and we can still hear the echos of the people jumping out the penthouse windows from the .com boom so its a sense of fear of sinking a house kinda money into something that could change so quickly.
2, Lack of Seed Money - This is the biggest issue for most, so you scamper around in the free to reg while you make the money to buy, then you move up the chain. This creates a comfort zone, if you find a place that puts food on the table, why risk climbing ?
and my personal fav.
2.1 - Building your seed money but not liking to sell. I HATE and I do mean hate, its on parr with people who think because there is markings on the road that pedestrians should know they are turning not going straight on, so they don't need to indicate, since cant you see the damned arrow 25 meters away on the road!
Also on par, with f'ing people who walk SIDE BY SIDE with trollys in supermarkets chatting about what soap character is humping which other beast and a speed so slow that Jenson Button thinks he's stuck in a time warp, and you can't get passed them.
Also along parr with parents who have kids that go ARRRRRRRRGH every 5 minutes down your ear, that are so piercingly loud your dog 4 miles away pee'd itself.
I could go on to describe how much I hate selling my names but I think you get the idea