The Rat. A nightmare-inducing character in the original, we have an even more diseased and ragged-looking rodent with red eyes. The slightly uncanny CGI on the character makes it even scarier.
Older Than They Think: The dogcatcher being more malicious was actually first done in Lady and the Tramp II: Scamp's Adventure. In both movies, the dogcatcher is motivated by revenge to do his job. While the animated sequel doesn't portray him with much sympathy, the live-action remake has him respecting Jim Dear and Darling's decision to adopt Tramp.
The Adaptational Wimp involving Trusty mentioned on the main page has not gone down well with the character's fanbase.
While it was inevitable given the Values Dissonance factor of their appearance in the original film, Si and Am are two of the more popular and iconic characters of it, so the fact that they were replaced with Devon and Rex as well as the absence of "We Are Siamese" has left a bad taste in the mouths of the fans who love them and the classic song.
Unintentional Uncanny Valley: The CGI on the animal characters has had this feel to some viewers. In particular, the Tramp's appearance in the first trailer (in a shot reminiscent of The Shining, no less) really left an impression. It works considerably better for the Rat, since he's meant to be frightening.
