The book has a lot of variant rules, but it really hinges around two concepts. One is that there are 20 spell levels, corresponding to character level. This is both a big change (because it has a lot of ramifications) and a small one--because once you learn how simple it is to convert back and forth, it's easy. Basically, you just double the existing spell's level, and if it's a weaker spell, you subtract one from the result. So fireball is now a 6th level spell. This book, of course, does that work for you, providing a framework from the core rules. However, in so doing, I found I had to flesh out the spell list with new spells. The book contains a number of spells from Complete Book of Eldritch Might, Book of Hallowed Might, and AE (converted to core rules). I know some of you have those books, but not everyone does, and I always want the system to be complete. Plus, I created a lot of entirely brand new spells. Lastly, I changed some core rules spells and presented all new versions of them, just because after all this time I think a few of them needed a little work.
The other concept the changes center upon is to allow characters to go longer without needing to rest. To that end, recovering hit points is a bit easier, and spellcasters have so-called "disciplines" that grant them minor powers that are not expended the way prepared spells or spell slots are. In other words, once the wizard casts all his spells, he's still got a few things to do. As an interesting added effect, this also means that a cleric need give up none of their spellcasting resources in order to heal his friends.
If you're playing d20, I hope you'll check it out. I must say, it's been kinda fun to stick my toe back into game design.
