Questions I always ask myself when designing icons:

1. Does it scale well, especially when making it (really) small? Are details still visible/recognizable?

2. Does it work well with 1 or 2 colors as a max?

3. Is it easy to differentiate the icon from others / is it easy to recognize what the icon is meant for?

If any of these questions is answered with a "no", I would not consider the job to be done, just yet.

http://psd.tutsplus.com/articles/7-p...e-icon-design/