Both BFA and now SL are very clearly less popular than Legion and this reflects my own personal opinion.
I was subbed for the entirety of Legion ~2years. BFA just a few months and SL I just resubbed but can't bring myself to log in or care, even with a new patch on the horizon.
A few things stand out to me, but curious what everyone else thinks:
- A new class, the iconic Demon Hunter
- A simple, but nostalgic story hook. Legion was a generic big bad and finally returned
- It took place on Azeroth (so did BFA, but SL feels... "out there")
- M+ and WQ were just introduced. Dungeons and the outdoor world were previously completely obsolete after reaching a certain ilevel. M+ and WQ now are considered boring and tedious, but is that because the quality has gone down or really just the novelty wore off?
- Heavy class focus made alts appealing. You basically had a uniquely flavored experience on each class with it's own mini-story and perspective on the larger conflict, as well as a chance to finally get some class specific mounts, titles and cosmetics.
- The entirety of the end game systems felt holistically intertwined: Class Hall + Class Campaign + Class Artifacts.
- Artifacts had a ton of cool challenge, hidden, PVP, etc looks to collect for 36 different specs over the course of the expansion