You also really want to take the time to think about how the market is going to evolve.You need a market that's going to be big in 10 years.
No matter what you choose, build stuff and be around smart people.
You can basically change everything in a startup but the market.
The hard part is that this is a very fine line. There's right on one side of it, and crazy on the other.
You need conviction in your own beliefs, and the willingness to ignore others naysaying.
At YC we have this public phrase, and it's relentlessly resourceful.