Going off memory here so I might be off a bit but the main differences with the new one are slightly faster CPU w/ more cores, higher resolution display (1920x1200 vs 1280x720), more RAM, different dimensions (narrower, taller, thinner), an added notification LED and rear 5MP camera, and a newer version of bluetooth.
So far my general impression is that all the little things add up to a nice upgrade, but nothing worth getting crazy over if you already have a first gen. I'd still use either of them happily, and the first gen just got the 4.3 update yesterday, so they're both still cutting edge in that respect.