Startups fail for a variety of reasons, PG listed some mistakes that Startups make in 2006[1] which is still actually a good read/relevant today (I disagree with the Single Founder point though).
However, there are problems which you generally see when a startup fails:
1. Poor Management Team
- Can't effectively execute, have a poor strategy and a poor management team 'breeds' a poor team (A players hire A players etc). A good management team will avoid the other issues below:
2. Market Problem - No Market or a Market not large enough. (May have been too soon etc)
3. Product Problems - Creating a poor product, so noone uses it.
4. Business Model Problem - Acquiring a Customer costs more than Long-term Value
5. Poor Execution
6. Building A Company around Technology not the Solution - This goes back to point 2 but your users don't care about your stack. Seriously you users do not care about your stack they care if you have a solution to their problem. If you do then you have a potential company but if you focus on building it around technology instead of giving them the solution, you're going to run into a lot of problems.
7. Lack Of Focus
- Not saying 'NO' enough
8. Running Out Of Money
- Most people raise to get to the next round of funding. That is wrong! If you are raising money etc then you should raise enough money to achieve a set number of milestones which attracts another round of investment from existing/new investors.
[1] http://www.paulgraham.com/startupmistakes.html