Definitely any sharding that exists beyond the first weeks in the starting areas. If I can’t avoid it by not playing the first month, I won’t play ever. If I’m in a location, I should see everyone in that location on my realm without having to group with them to share a shard.
Stuff I don’t see being likely, but if it suddenly became part of WOW Classic would see me walk quickly the other way:
Plans to cycle servers (fresh servers, rolling restarts, ANYTHING that makes getting to level 60 not a permanent thing).
Achievements and/or Collections.
Dungeon maps and boss tracking (showing where they are or which have not been killed yet).
Any form of quest tracking in-game. As the priority item that makes me want to play WOW Classic is doing the horde questing I missed in 2006, that’s the area that needs to be the most authentic.
Modern character appearances as default.
Any cash shop. The moment someone at Blizzard even thinks “we’ll just add the cash shop to offer this pet”, I’m done. I get it. People will play Retail and buy tokens for game time, but other than getting that game time it should not touch WOW Classic at all.
Changes to classes (talents, gear, anything non-vanilla). Though it might be surprising that I’d be okay with raid tuning, if I thought the WOW Classic team actually had a 2004 MC reference to work from. The difference is raid tuning is about making a group experience more authentic, while class tuning would be about making individual experiences less authentic.
The other thing, that I won’t “quit” over, but that will keep my play time in WOW Classic relatively brief, is no TBC Classic ever. Paying US$15 a month for only WOW Classic (because I can’t imagine them making Retail a game I want to play again) won’t hold its value long. Paying US$15 a month for both WOW Classic and BC Classic would feel like a much bigger value, and keep me playing a long time.