Here are the top 18 reasons that kill startups:

1. Single Founder
2. Bad Location
3. Marginal Niche
4. Derivative Idea
5. Obstinacy
6. Hiring Bad Programmers
7. Choosing the wrong Platform
8. Showness in Launching
9. Launching Too Early
10. Having no specific User in Mind
11. Raising too little Money
12. Spending too Much
13. Raising too Much Money
14. Poor Investor Management
15. Sacrificing Users to supposed Profit
16. Not Wanting to Get your Hands Dirty
17. Fighting Between Founders
18. A Half-Hearted Effort

More information on http://paulgraham.com/startupmistakes.html