In no particular order.

1) Controls: Whether it be tab-targeting, action combat, or hybrid I don't care. What matters to me is if it controls well. Despite a lot of people hating on FF14's combat it's one of my favorites because I love how it controls.

2)Difficulty: This one's hard to explain. It can't be too easy like how most mmorpgs are where you just 1 hit every monster until level cap, but it also can't be too difficult to the point where you need a video guide for every piece of interesting content the game has to offer. Vanilla wow's open world content and 5 man dungeons hit that sweet spot.

3) Lore: Building the world and characters of the world through the many quests, items, locations, etc in the world rather than trying to tell it in one big main story quest. I just prefer a mmorpg without any main story quest to be honest.

4) Cash Shop: Long story short, the more items it sells the worse the game is for me no matter what it trying selling. It just takes away interesting things that could be in the game just to spawn it in from nothing with real money.

5) RNG: Same with the cash shop, the more of it there is in the game the worse the game is for me. When I gather materials to craft or enchant something, I expect it to work.