Well, this isn't exactly the question you want to ask - you'll end up getting a ton of text book answers. However, basis my experience of building products of our own and a few for our clients, these are a few checkpoints that you should tick -

You enjoy the work, and/or your new business complements your existing setup.
You believe in the idea, and know the problem you're solving. This will help you calculate or estimate the potential business size in the future.
You're true to yourself - there are some people who're good at operations, some at strategy and some at management. Know where you belong and trust others for the rest.
Check scalability, if you're into a startup mode. Use technology to solve problems.
Never compromise on team building. It's the key to success, every time you hire, your potential should grow 2x.
Be ready to devote 100% concentration on the final product, always. There will be a lot of people who will try to deviate you - but that's the milestone you want to hit first.
Know what your ultimate strategy is. Are you looking to build a national/global business or are you creating a cash cow or do you wish for an acquisition opportunity.
Be ready to listen more than talk, and improve at all times.
Always account for variable change.
Be ready to get your hands dirty, nobody is coming to you with a bag of cash otherwise. You need a good business, you need to slog for it.