As said in the title: every expansion that does not add a new class is crap, period. I don't care about the balance of the game, or how pvp gets more f***ed because of adding a new class and that there are so many classes already in the game blabla as some people even here in the forums always argue if blizzard should add more classes. It's not a surprise that not having a new class on a new expansion makes the game much worse and as we saw it in the history of WoW, every expansion that didn't add a new class was guttercrap.
And don't even begin with TBC; tbc gave both factions a new class; many didn't played the other faction back then, so many players actually got a new class that they never played before.
If blizzard wants to make the next expansion somehow successful (if they don't wrap everything up for a wow 2), they need to give us a new class. It does really not matter if the balance is bad (at the beginning), or what class they bring out, just that they need to bring out a new class. Be careful: Having a new class will miraculously not fix a bad expansion, just that having no new class will make the expansion bad.
Having a fresh class also bing a fresh air to the game, even if you are playing the same game, having a new class always brings a new way to play the game, not only for the player who plays the new class, but for existing classes too. Playing the first time healing a Demon Hunter was a new experience too, even if i didn't touched the class yet.