Honestly, I noticed the same thing in BC. Just, not with vehicles, but with both flying mounts and the new races.
In The Burning Crusade, flying mounts were kind of like the airship system at the end of Final Fantasy X. You could use it to go back to places you had missed before easily without lengthy backtracking. And you could open up new areas with it too, story-driven or just bonus levels and items. Blizzard loved this feature, but it was never actually required, except for a few times.
And I honestly think Blizzard regretted the decision of putting them in in the first place. I often wondered why Blizzard used the whole "NO FLY UNTIL 77" thing in Northrend. They claim it's because they want people to appreciate Northrend and not just fly around all the time. I agree with that, but I also think that while designing WotLK, they kind of thought, "Well, they can't just fly around to all the quests, that'd get very boring, very fast. Why'd we put that thing in in the first place? It's not like we can just make them usable in Outland only. We have to give them back some time. I guess we'll just put a level limit on it or something."
And as for the new races in BC, I feel like it was Blizzard's way of saying, "Hey, you wanted new races? We got new races! Look at how shiny they are!" In almost every town or quest in Outland it seemed like, the new races played a major part. Which I guess makes sense. I mean, the Blood Elves were trying to start all over again and rebuild from scratch, and the Draenei had already been in Outland for a while before. Oh, and they just happened to crashland on Azeroth right before the opening of the Dark Portal.
But it seems that with the exception of the Orcs, Draenor being their homeworld and all, the other races really had no major reason to be there. Gnomes? Tauren? Another planet? Sure, why not? It seemed like Blizzard liked showing off their new things a bit too much, and pushed all the old stuff to the side, similar to how Old-World dungeons are mostly forgotten except by levelers.
And to me, vehicles are just another way of Blizzard showing off their new toy; their new favorite feature. And I think they may have overdone it a little. Or a lot. One or two boss fights with vehicles is fine, and the Strand of the Ancients is a good idea. Wintergrasp too. But the amount vehicles play in the expansion after being non-existent in the past two incarnations is both too sudden and overdone.
