The changes are split between two groups: playlist and patch changes. The playlist changes are numerous, and really help to balance and perfect the online experience. Maps are being added to rotations, new playlist variations are being introduced, and some rules are being changed. As for the patch, there's an awful lot of network optimization, the addition of host migration (your party doesn't get booted if the host takes off) and, perhaps most importantly of all, Kill Cam view has been added to everything: claymores, RPGs, grenades, even fracking Air Strike Bombs ("follows bomb from plane to target" ... oh yes).
The only thing we don't like about this updates? There's no ETA outside of the "month of December." Hit that 'Read' link for the full breakdown and start counting down the days.
Update: Infinity Ward has clarified some details about the playlist updates and patch.