1.  Well before joining a startup you should be sure of two things, you believe in the product, and you believe in the founders.  
2.  If you plan joining a startup , plan on being there for at least two years, before you decide if it will pan out or not.  
3.  Expect to work a lot more than you probably are.  
4.  You're current job won't be as hurt as you think, believe me life goes on at every company, and you're leaving won't hurt as much as you think and should not be part of your consideration.  
5.  If you really want to work on something innovative to expand your skills work on your own product that interests you on your free time.
good luck