Those who are successful look for a couple of factors:

SCALABILITY
Is this something that can/will reach the masses? Will the masses "need" this product or service?

NICHE
Is this product or service something that fills the need for a few, and provides healthy margins, as a result

ROI
How soon can I get my investment back, and how much will I earn for my efforts?

EFFORT
Slightly different than ROI; how much work will it take to get this to the masses, or the niche markets, who need this product? How much time and energy will it take to develop/manufacture/market/ship?

I hope this helps, feel free to setup a call if you have a specific idea that you don't feel fits into this metric.