From a Yahoo! Article about the changes:
Definite Changes
Fewer Trainwrecks
In an effort to bring some more "credibility" to the show, fewer freaks (Mary Roach), delusional dorks (Michael Sandecki), and hot messes (William Hung) will be featured during the auditions.
Character Development
With Hollywood Week finally becoming Hollywood Weeks, we'll actually get to know the contestants and their stories, unlike in years past when the Top 12 could easily consist of strangers.
Wild-Card Round
For the first time since the show's second season, the judges will select the final three contestants that will comprise the Top 12.
Top 12
With the semi-final round being comprised of 36 contestants -- as opposed to the usual 24 -- and with the judges determining the wild-cards, the makeup of the Top 12 won't necessarily be split evenly among males and females.
New Judge
The new kid on the block is songwriter-producer Kara DioGuardi, the mastermind behind Xtina's "Ain't No Other Man," Gwen Stefani's "Rich Girl," and Pink's latest hit, "Sober." Will she rely on her professional experience like Randy? Will she battle with Simon? Will she ultimately replace Paula? (All parties deny this rumor). Will she tell it like it is and risk hurting the contestants' feelings?
Shorter Results Show
Wednesday night's results show will be pared down from one hour to 30 minutes.
Rumored Changes
Fewer Themes
Rumor has it that viewers will no longer have to witness pop tarts trash country classics, crooners butcher Broadway tunes, and divas destroy Big Band standards.
Younger Mentors
In a perfect world, Season 8's finalists won't have to pretend to know who Neil Diamond is, let alone sing songs from his infinite oeuvre. Same goes for Andrew Lloyd Webber and anyone else with liver spots.
I can't wait to watch them go down in flames! 
