To many, myself included, this characterisation felt dishonest to the original. In fact, the most common complaint against David Tennant's Doctor, from those who have seen significant quantities of Classic Who, was that he simply didn't “feel like” The Doctor for explicitly these reasons.
Now, I'm sure at this point you're wondering what this has to do with The Night of The Doctor. The connection is simple. Steven Moffat attempted to explain this drastic, and jarring, change in his personality. Simply chanting “Time War,” referring to an off-screen plot device rather than showing actual character growth is lazy and unsatisfying, so Moffat actually came up with an on-screen reason for it. The elixir that Eight drinks, much like the crack in time that erased the ill-conceived (and appallingly designed) Cyberking from history, spackling over these cracks in the canon. It explained the personality change – The Doctor had to become this darker, more aggressive man to fight this Time War. He had to literally become a different person. It stands to reason that in the next regenerations, this would gradually wear-off and he'd become more like he was. While I can hardly explain why Ten was infinitely less Doctor-like than Nine, it fits perfectly that Eleven, possibly after seeing how far his “War” persona took him as Ten, forced himself to regenerate in such a way that brought him more in-line with how he used to be. The fact that Eleven feels many times more like The Doctors of the Classic series actually makes a good degree of sense with this in mind. It will be interesting to see how Capaldi takes on the character, keeping this theory in consideration.
So Moffat had this goal in his script already. But he also aimed to seek a gap in the show's existing canon – to provide a regeneration for Eight. While it's hardly the best regeneration in the show's history, it is definitely better than some of the worse ones (Six, Seven, Ten). One couldn't provide a satisfying regeneration for Eight without a lot of build-up and, for what it's worth, Moffat does try to provide that by referencing, and making canonical, his many years of Big Finish audios. This isn't as effective as it could possibly be, but Moffat made Big Finish canonical and I am quite pleased at that fact (as, I imagine, he is as well. Being an immense fan of their work himself).
The question now is where Eight goes from here. There are petitions online to get him his own spin-off series. If this happens, it will be curious to see where they approach it and who is put in charge. I, for one, and firmly in the camp supporting a spin-off, as I believe Paul McGann's physicality can lend a lot to the series, but only on the condition that they do not attempt to recast existing Big Finish companions. Just as Karen Gillan was almost rejected for not being “pretty enough,” I fear India Fisher may get recast for not being a stick. What's more likely, however, is that Eight will get an all-new companion developed by the head-writer of the show. I would be thrilled to see this, provided they don't go and attempt to inject another canonical romance into the series. It's weird and we already have enough wars being fought in the fandom over Rose/River/no-one as-is. We don't need any more.