Character switching and being able to play as any duo of characters, also gets rid of lives in favour of the five characters just having one life (but you can gain characters again from power up TVs). The levels are slightly reworked to allow for Mighty and Ray's new abilities, and they have a new colour pallet.
Oh, that sounds interesting with the lives. Reminds me of of Subspace from Brawl with choosing characters and having to switch if one dies. I still love running through this game, so having new reworked levels and this system sounds like a nice shakeup. I've been saving up my Nintendo gold coins for the dlc, and it's gonna only cost me like ¢50 with them.
Edit: I believe at one point it was mentioned that using the super peel out or insta shield will be available for saves after this dlc right? I'm a huge mark for the super peel out since Sonic CD is my favorite in the series.
Edit 2: just watched that ign review. That was an awful review and all I heard was, "it was too hard, not enough content" even though the dlc is only $5. When did they review dlc for games anyway? I felt they wanted to shit on the game and used the dlc as a reason to do it.