I can see why people might be sensitive about the title of this programme, but I honestly don't think that the use of the word "special" was meant to refer to people with special needs, it was just a brief description to indicate that it was a feature length, special episode. Having just watched this programme, it wasn't themed around Christmas so could not be called a Christmas Special, and if they had reversed the order of the words to read "Special Derek" that may have caused a lot more offence.
I enjoyed this special edition very much, although there were a few cringeworthy moments, but they were not to do with the character of Derek, who came across as a likeable, caring person that many people would warm to. I'm unsure if the main character was intended to be a person with Asperger's syndome or have special needs, but even if that was the intention, I see no ridicule or mocking of the afflicted, more a very clever characterisation which makes the viewer see things through the eyes of a gentle soul who struggles at times to understand the world around him. IMO Gervais has done a good job in creating the main character, and it's clear from this particular storyline that Derek is a character who others like and respect, for his kindness and humanity. The comedy aspects stem more from the other characters, who are anything but "normal", whatever that is.
The series is not exactly a comedy, nor especially dramatic, most of the time, but it has really funny moments at times, combined with some serious, quite poignant moments at others. I think Gervais has done an excellent job and for those who took offence at the title, I'd suggest you watch this episode right to the end, then you might see that Derek is actually shown quite a lot of respect in this special episode.