Well 2.1 has CDMA compatibility built into the OS. 1.5 is patched to allow CDMA which is one reason why there are so many problems with Android on CDMA.
Basically, my battery went from like 9 hours on 1.5 to probably lasting me a day and a half if I needed to. I charge every night anyway, but I think it would go that long at least.
You get the full Market, all the apps we couldn't see in our 1.5 market because they were written for 1.6 or above. I have come across a lot of them, like Urban Spoon, which I never could find in the 1.5 Market. Also, the Market is new, polished, better navigation and so on.
The OS is fast, very fast. I was lucky to not have problems with my stock Hero, but once rooted and running Fresh 2.0d, it is like night and day. Way faster. The rest of the OS seems more polished too. Menus seem more lively with color and so on.
The weather widget, instead of just showing the animation in the widget it is now fullscreen. There are a lot of little things, but basically it is just a better overall package.