I'm still downloading a few apps but I'll put down what I have initially noticed.
1. CC apps are not installed over CS6 apps, CS6 remains on your disk and is still usable.
2. Premiere Pro CC and Adobe After Effects CC projects are not backwards compatible and will not open in CS6. However After effects does have an option to save as a CS6 compatible copy.
3. After Effects CC is still not optimized for retina macbook pro.
4. Photoshop CC files will open with no problem on CS6, CC-only smart filters that are applied to smart objects are still preserved when you open in CS6, you just can't modify the smart object or add/change the smart filter properties (you will see a triangle with and exclamation mark on filters that are not in CS6)
5. Premiere Pro CC is referenced as 7.0 After Effects CC is 12.0 Photoshop CC is 14.0 Illustrator CC 17.0
6. Illustrator CC files can be opened in CS6, you are warned about potential loss of data on opening and the file in CS6. The quick document I tested opened in CS6 with my objects inside a document-sized clipping mask that it was not saved with in CC. CC still has the ability for backwards compatibile saving all the way back to Illustrator 3 (not cs3).