It really is a lot more than the hair. In DotD, DT looked puffy and pasty. Take a look at the pics below. One is from DotD, the other is from the Muppets sketch - both are from a fair distance away, and his expression in both are similar.
See the difference in how he looks, ignoring the hair (which you can't really see in the Muppets pic anyway)? In which picture does he look older? And I deliberately picked a "good" picture of him from DotD - there are much worse ones, in which he looks ten years older than he did when he regenerated, not the mere four years older that he really was. In the Muppets pic, he has five extra years and harsh stage lights - which exaggerate imperfections - and yet he looks younger there.
It's all a matter of makeup. The makeup artists can make an actor look like whatever they want, and we can see from the Muppet sketch that they could have made him look very similar to the way he appeared when he was the Doctor (especially in series 4 and the specials), but they chose not to.
Here's another thing to look at: https://www.youtube.com/watch?v=rjBswS_1nq0 This is an interview from two years ago, when he was clean-shaven for Richard II. If he could look like this for an interview, the BBC makeup department could have given us Series 4/Specials Tenth Doctor.
I keep meaning to write that analysis of DotD that I promised you like two years ago, alumfelga. I have a bunch of it done, but it's not in any postable form yet. And I think, serenityslady that you'll find that I pretty much agree with you. I honestly feel that Ten was there because it was expected, and so he was given nothing to do except comic relief. He had no actual contribution to the show, and frankly, his part really was an insult, and so DT didn't bother to give it his all. I also feel that the script was written before it was decided to try to include Nine and Ten, so when DT agreed, his character was stuffed into the already-established Zygon storyline, with Queen Elizabeth substituted in for whoever had already been there. If you change QEI to someone else, say some queen known for having side romances, you can see that what he did - seduce a woman with an ulterior motive and no regard for her at all - was exactly an Eleven thing to do.