We have been most years since 1995 and I didnt notice anything wildy different when we went in December.
There were always good and bad CMs at Disney, most of them being good, if not brilliant. I encountered the usual mix at Christmas.
Never noticed a problem with cleaning (try DLP - now they DO have a problem!!).
Not much in the way of new rides recently, but there are some headliners opening this year and next (Soarin', Motors Stunt Show, Expedition Everest, a new water ride at Typhoon Lagoon to name a few). Also, a lot of stuff has been in rehab, which will give some of the older rides a lift.
A lot of the US DISers visit a lot of times in one year (for long weekends etc) and I think any small problems are highlighted more by these short breaks. Memories are very fresh from last visits, hence even a small thing that isnt as good as before seems a bigger problem than it probably is.
There was a lot of bad press about the new Stitch ride at MK (replacing the old Alien Encounter), but not every ride is going to appeal to every person. Lots of people loved it, lots hated it.
My advice is to go with no preconceptions. Enjoy YOUR trip and your experiences. Dont not do a ride or attraction because you read somwhere it was no good. Experience everything the World has to offer and I pretty much guarantee you will come away loving a lot more than you hated!
Have a fantastic trip!
Allie