Regarding specifically the idea, it has been done before (Diotoir i believe was spring powered) however it does have its potential problems. Namely the slow return, and also being the least powerful of the common mechanisms.

What you get for that is simplicity, uniqueness, and ease of continued use (once built). Also its something you can just charge up and play, which is always a bonus when starting out. Only potential problem is because its so rare, it may be difficult getting help unless you post an idea and get it "double checked". But certainly at the end you could have a very serious robot which is easy to use and maintain.

Oh btw, you may be thinking of featherweight rather than lightweight.