Math, lots and lots of math. The new chip has an actual floating point unit instead of it being emulated, support for the NEON SIMD extensions, both of which will definitely help in speed in dealing with any amount of number crunching. It's also got virtualization extensions, so you should be able to run Xen on it and do some level of OS development if you wanna get one for that. Really, it boils down to a lot of little differences that will line up together to make a lot of tasks on the Pi a lot easier to handle.
The previous RPi did actually have hardware FP, see http://raspberrypi.stackexchange.com/questions/545/does-the-.... I think the arch support is probably the big issue - any standard Linux distro with an armhf port will work on the new RPi. Probably gets you access to a lot more interesting Debian packages than you did with Raspbian.
Thanks for the correction. I recalled the original release of debian for the Pi didn't have floating point support, but did not recall that that was because Debian was building towards the least common denominator. I don't think you'd get that many more interesting packages with the new arch, but you probably will get a lot more usability out of the packages
