David Sirlin has an excellent blog. I read it as often as I can, which isn't as often as I would like to.
I was going to make a lengthy post about the Diablo 3 skills and abilities system. How well-designed it all is, how easy to use and customize it is, how easy to understand it is...basically how elegant it is. It really should win some sort of award, if any such award exists.
However Sirlin beat me to it by quite a bit. He says it better than I ever could. Go read his blog. You will thank me later.