Until Series 4 of NuWho, one thing that was always the same about the Doctor was that when regeneration occurred, he would have a new face, personality and body type, albeit one that still wasn't ginger. That all changed, however, when, after being shot down by invading Daleks, the Doctor was able to regenerate enough to repair the damage, then siphon the remaining regeneration energy (the part that assumedly would have changed his appearance) into his "handy spare hand" that he conveniently kept in reaching distance aboard the TARDIS. No other Doctor before or since has done anything like that, and it just goes to show that the Tenth Doctor was far too full of himself to move along and let someone else take the reigns for a while. On top of that, with the subsequent creation of the metacrisis Doctor, he wound up losing Donna Noble, who is many fan's pick for "Best NuWho Companion Thus Far", as well as giving his BFF Rose Tyler her happy ending as well. It was a very long day.