Defining criteria to evaluate ideas can be difficult. Best bet is to rely on the goals of the overall project, or goals for your team, and apply them. If you’ve chosen the right things to brainstorm about, there should be a decision you are facing or problem you need to solve that you hoped brainstorming would help you to understand. If the brainstorming session didn’t provide anything useful, it could be you need to refine the question or problem, and try again, or that brainstorming is the wrong forum for it. It might be better to ask people to work on the problem on their own (which might be difficult if they have other responsibilities that you aren’t willing or capable of eliminating) or consider brainstorming with a smaller more intimate group. With just 3 or 4 people you can run faster, change directions more easily and go deeper into problems that you can with 8 or 10.