Lots of extra features -- you can make/download custom maps, you can download Garmin's proprietary "Bird's Eye" imagery, support for paperless geocaching, ability to turn entire map sets on/off, wireless "Chirp" and exercise interface, lithium battery support as you mentioned, high-speed USB interface, etc.

I used to have a GPSMAP 60Cx and I would consider the 62s a relatively big improvement in terms of overall features. As far as GPS reception & performance, I think they are pretty close.

Realistically, I hardly use the lithium battery option, but it's nice to know it's there. A set of the Sanyo Eneloop NiMH cells work really, really well.