del Toro left mainly due to scheduling issues. The Pre-Production for the Hobbit started as far back as 2009, which was when MGM was in the middle of pretty much declaring bankruptcy. After too much waiting, del Toro stepped down, and instead of looking for someone else, PJ took the helm.
Del Toro's script was very different, not only that, but the art direction as well. Though Alan Lee and John Howe were still involved, they were definitely taking a bit of a different route to set it apart from the original Trilogy. From what I know, there are a few things in del Toro's version that were very interesting, while others would have been a huge risk that in my opinion would have upset many.
The decision to make a third movie wasn't made until principal photography had already finished. This is about 6 months before the release of An Unexpected Journey. The studio knew Jackson had enough material to stretch the first movie, he only need a few pick-ups here and there, and they would shoot the rest between movie 1 and 2. This is where things kinda turned ugly for the production in my opinion. Not only because of the obvious cash grab, but I honestly think Jackson didn't want to do three movies, but 2, with really long Extended editions for the fans. But in a movie like this you have to play by other people's rules as well. Something Jackson has always mentioned is that he knows he's not only responsible for fans of the movies and the books, but that he is also responsible for those who's money is at stake (del Toro is very much against this, so maybe he had foreseen something like this would happen which encouraged him to exit.)
If it would have been two movies, the first movie's climax would have been the barrels in case you are interested. Also, Tauriel was originally thought as a character for the movies, but the love story wasn't Jackson's original idea.