People forget that during it's time Vanilla was the most casual friendly mmo on the market.
Some casual players might not like slow leveling but I think it might also affect the hardcore ones. Hardcore raiders were the reason Blizzard speed up leveling in the first place so it's easier for them to have multiple classes ready for raiding. I enjoy leveling, and I take my time with it on retails so I take my time with it on Classic.
LFD, LFR and LFG this might anger some people but dungeons in vanilla were completely different thing and were aimed at more hardcore players, the same goes for raids.
Gear quality well, players that remember the olden days of gaming won't be mad but the more entitled casuals from today will be furious they can'g get their purples.
More problematic for a casual player would the fact that there aren't that many flashy mounts to farm. You pretty much gonna have to use your racial or class mounts.
Respec cost also is something that will affect more hardcore players than casual ones. Casual player doesn't change their build 10 times a day usualy while more hardcore one would want to have the best build for the situation and role.
No transmog but you can just dress up with cool looking gear to show of if you get some.
Casuals could just farm some sets, or do professions. If you don't have a lot of time to play you will have leveling to do so that can keep a casual busy.
Also there's no flying and we all know how people complain about no flying nowadays.