Sure, John, I'd be glad to. Off the top of my head:
1. All the classes are different. Instead of the PhB classes, we have the Bully, the Dreamer, the Jock, the Nerd, the Normal Kid, the Outcast, and the Popular Kid. Classes are 6 levels and receive feats at 1st (one standard, one origin), 2nd, 4th, and 6th.
2. The skill list is much shorter. Some skills are combined (Listen, Search, and Spot become Notice, while Climb, Jump, and Swim become Athletics — then there are new skills, such as Nimbleness and Puzzles). Lots of new feats, too (most of which are designed for
Grimm).
3. The magic system is different, to say the least: incantations can be learned by anyone, but entire schools (Necromancy and Evocation) are barred from PCs.
4. Imagination points. Similar to action points, but more flexible, this is one of the things the book describes better than I could (at least if I were trying to be brief).
5. Rules for grappling, being swallowed whole, and despair.
The biggest changes are in the classes and skills — pretty much everything's different. As for what my game would be like, I'm still working on it, but I know that (in addition to the Grimm Fairy Tales) Alice in Wonderland and the Wizard of Oz will both be influences, and there will be six chapters. I have other ideas, but it would spoil a lot of the campaign if I were to drop them out here.

I'm thinking about simplifying combat a bit (we won't use maps, for one, and we may or may not use AoOs — I'm still thinking about that), but that's the only mechanical change I'll be doing as of right now.
Hope this helps.
Nick