It varies. I'd be lying if I said mood wasn't involved. But I ask questions like:

  1. How responsive are the startup founders when I e-mail
     them about problems?

  2. How portable is the work I'm going to put into the 
     site? Data portability is actually probably my number 
     1 concern.

  3. Does the service give me value that I couldn't 
     create with my own server after an hour of coding? 
     (Why would I sign up for yet another contacts service 
     when I can host my own LDAP server for free?)

  4. Is there a vibrant community supporting the startup 
     from the get to?

At least these are the first that come to mind.