Joel Spolsky blogged about this. His main points were to look out for:
- A clause which forbids you to hire anybody who works for the company that is making you sign the NDA
- Non-compete clauses in their employment contracts
- Another dangerous clause says that you agree not to hire, or cause to be hired, anybody from the company if you leave for a period of x months (usually 12 to 24).
I've also seen contracts that forbid the hiring of working for customers of the company. If the company hiring you has a lot of customers, that could be a large amount of jobs you couldn't take after leaving.
All in all, I'd look out for clauses that restrict employment in general. For the most part, the goal of these clauses are to protect trade secrets, but they could also seriously hinder your job search.