I love the startup experience. The people are great, the possibilities seem tangible and success is always just over the horizon. I've met the best and smartest people I know in startups. But the reality is that most startups don't succeed. Most entrepreneurs will admit that there is a huge element of luck in a successful startup. A startup needs that elusive combination of a product or service that can't be easily duplicated, that either solves a pressing client problem or is something people can't live without, a reliable and scalable infrastructure for delivery and support, and a roadmap for constant improvement. Even with these things in place, success can be undercut by problems in the external marketplace (like a global recession) or by other factors beyond anyone's control. And by nature entrepreneurs are optimists and always see success just one client or one release away. Go into a startup for the exhilaration, education and excitement, but be realistic. When it stops being fun, it's just like any other job.
One situation to be aware of is the zombie startup - a company that should be dead but keeps on living. Click on the link to see how to deal with one.