Well, the Campaign Setting book for Dark Sun is for both players and GMs to read, (they didn't come out with a Player's Guide for this one,) and after a quick flip-through, so far it's had some interesting additions.  Themes, for example, which you can take at the start of play and acquire an extra encounter power, and then as you level up through Heroic tier, you get a selection of powers that you can take in place of your regular class powers.  These themes have certain Paragon Paths tied to them, as well, and there are actually a total of 22 new Paragon paths in the book, two for each theme, and then one each for the two new racial choices.<br/><br/>There are new backgrounds, notes on how the different races fit in together, class options for the Shaman, Battlemind, Fighter and Warlock, new feats, five new Epic Destinies...  Oh, and Wild Talents, which are like Psionic Cantrips. ;D There's plenty of cool things, over all, and I'd probably need you to ask about something specific in order to give you the level of detail you're looking for.