That's why I have serious respect for anybody that makes it work more than once, those are the real entrepreneurs. For the rest it really is mostly luck.
I've scored pretty good with my first 'real venture' and I can testify to how unbelievably hard it is to do it again. So far no success :) But that won't stop me from trying very hard.
One thing definitely has changed in the last 10 years, back in '95-'99 most things you put out on the web were operating in an almost competition free arena, some of the stuff we got away with back then we certainly wouldn't be able to do today. Users expectations are a lot higher than they used to be and there are a lot of parties on the lookout for good ideas so if you 'launch early' with an incomplete product you just might give someone the last bit of the puzzle they were still missing.
It's a tricky situation and there are no easy solutions to any of this.