Haha was very prepared for this question. The main differences for me are that roots doesn't rely on ruby, it's built for advanced devs (starts with jade, stylus, and coffeescript), it uses roots-css rather than compass, and the project structure is a bit lighter and cleaner.

I tried yeoman and played with it for a while when it came out, but for me it feels vastly different to actually use. The concepts are definitely similar though. Great question!