What's a good idea?

Regarding the competition: if a competitor is doing a good job, but there are still customers out there waiting to be found, then replicating their technology and capturing that unfound part of the market would be a good idea. Try to get a feel for how many people want that product/service. See the following for links on the infamous parrot ebook:

http://news.ycombinator.com/item?id=520887

http://news.ycombinator.com/item?id=516215

I suggest delving into a subject that interests you, but that isn't too difficult for you to both understand, and make a contribution to; e.g. focusing on cryptography itself would be a bad choice, but maybe you have a novel idea about how to use crypto in a P2P application. You can then apply your domain-specific knowledge in order to implement your idea faster and better.

Twenty bucks, same as in town.

To me, a great idea is one that is solving a big problem in a large market. But that alone is not enough because if you don't execute it, competitors can come in and perfect your idea to its full potential.

This problem usually tends to exist because nobody has solved it yet or has not solved it in a manner that is practical enough for the masses to pick it up.

The fact that your ideas are implemented exactly as you thought of them confirms that if the company that is currently doing this becomes successful your ideas are good.

If the company doing your idea is being successful it confirms the market exists and I would look for innovative ways to improve on their offering - or execute better than they do. You want a strategic advantage when entering the competition.

It would also help knowing what kind of team/resources they have available. So that if you show up and take market share how quickly can they respond?