Wow, you've got a ton of great and positive advice here. I agree with almost all of it, but would add this...

There are no shortage of ideas, and no shortage of businesses to build if you have the ideas. If the company/market you are looking at has a good implementation with market penetration and barriers to entry, and if you can't SIGNIFICANTLY make a difference, then maybe keep looking. Take a read of Rick Segal's current post (and my comment there). http://ricksegal.typepad.com/pmv/2008/11/features-of-the-wor...

So, your final sentence says it all... go develop, you'll learn a ton going through that process and maybe find more ways to differentiate and improve on the market.

Don't forget to let us know when you launch.