Characteristics That Make a Startup a Startup
startuporbust.com
startuporbust.com
The other differences all follow from that one. Startups do technology. They're usually product companies rather than consulting companies. They have exits (either IPO or sale) and give employees options.
Craigslist clearly didn't intend to be huge at first (it wasn't even sure it wanted to be a web app), and Craig has made it pretty clear that he doesn't intend to IPO or sell (though a big chunk was purchased, by EBay, I think). Aside from that, it does have the profile of many startups - small, technically oriented, programmer-driven, massive growth.
37signals fits to some extent as well - bootstrapped by consulting, but product oriented, technically oriented, programmer driven, substantial growth.
Neither companies seem to want a sale or IPO - they're happy to settle into a role as profitable businesses. At that point, I suppose they are no longer startups.
While these companies are the exception, the approach they advocate does seem to be growing. So maybe you call them startups and say that there are many paths - and the IPO/sale exit is more typical.
just my two cents
FaceBook's product is on the market, and it's profitable. It's obviously not "finished", as they're adding to it daily. Most people would call FaceBook a startup.
Microsoft Windows is on the market, and it's profitable. It isn't "finished" either: just look at Vista. And certain areas of Microsoft - Silverlight, F#, LINQ - are most definitely not finished, and nobody knows if there's an eventual market for them. Yet most people would not call Microsoft a startup.
(P.S. xkcd rocks)