That is exactly what I am fearing regarding mounts. Horses lead to stables, horse sounds, etc. It is gonna take up a a big part of the aesthetics in D4.
A computer game is a universe where all elements focused on will colour the atmosphere quite a lot, so I for one, am against the current idea of mounts. I don’t dislike horses in general, they add well to the atmosphere in certain kind of fictions, like spaghetti westerns, but not in Diablo. It will quickly turn Diablo into a jockey’s RPG.
The Crusader’s horse skill in D3 is different though, it is clearly more of a magical, temporary skill where the character changes form when using it, that I can live with.