A few words of warning: as the name implies, the software isn't quite done yet, which means it's likely to (at least) crash, and (at worst) break your computer (I doubt that's likely to happen, but anytime you run alpha software, it's a possibility). In addition, the model viewer is in no way created or condoned by Blizzard-- there's a chance (albeit small, I realize) that they could consider this using a 3rd party program to modify the game files. Again, it's not likely-- I doubt this program actually does any editing, just reading-- but the possibility is there.
Anyway, the Model Viewer can make for some pretty cool looks at content you haven't yet seen in the game (or haven't yet been able to examine closely). And the Model Viewer is also responsible for some pretty amazing fanmade videos, too. The new version allows you to access the Blood Elf and Draenei models, so if you're interested in seeing what's just around the corner with the expansion (and willing to use unofficial alpha software to do it), have at it.