Dance Studio is such a niche feature. The previous features weren't niche. They changed the game massively. CRZ for example, is a game changer. LFD, LFR, BGs are all 3 game changers.
Btw IMO transmog is also a pretty big feature. Its all in the eye of the beholder. If you want to see "one big feature in every expansion" you'll see only one. But I see various. The term game changer isn't something with an arbitrary set of definitions (correct me if I'm wrong) so pretty much any feature can claim it.