Ultimately, it’s just my favorite game of all time.
I enjoy playing with like minded people who also find a 20 year old game to be their favorite game.
I like knowing it isn’t really going to change (yes yes I know, some changes blah blah).
I love the content — the zones, the quests, the dungeons, the raids. The world is so immersive, something none of the xpacs made me feel.
I love the permanence of it. You never are leveling up past 60, so this is it. Your best gear is your best gear.
I love how long it takes (for me anyways) to achieve my goals in the game — be it leveling or getting my desired gear. I’ve only really leveled a few of the classes to 60 yet, so still lots more to do for me that would be new.
It’s also fairly casual, despite massive time sinks. I don’t have a lot of time to play week to week, but I have consistently played over the years. I like that I can just pop on and raid, or pop on and level a bit on an alt, or farm some stuff or what have you.
Most recently, I’ve loved helping to build a community on Grobbulus with a bunch of other likeminded weirdos who just love vanilla wow. I’ve played vanilla on and off since 2004, and somehow playing with these guys on Grobbulus and having a whole server to ourselves (one horde guild, one alliance guild) has been my favorite time in Vanilla. Took us about 2 years to progress from 5-10 people clearing MC to nearly 40 for our first KT Naxx clear (and we’re always looking for more friends to join!).
To sum it up — best game content and design, doesn’t change*, massive and immersive world, fairly casual friendly but also lets you be as hardcore as you want, great community of likeminded and dedicated players, it’s fun.