Blizz stated in a watercooler that they're using the 1.12 database as a starting point. I don't think it's a stretch for people to assume that most game features and mechanics will be in their 1.12 state. The kind of stuff I would expect to see reverted would be numerous AV changes, but I seriously doubt class mechanics are going to be reverted. I'd be surprised if the threat changes that ruin pre-naxx encounters are reverted.
Logging in for the first time, being welcomed by THAT login screen and THAT music... You can't not be taken back if you've played Vanilla, and especially if you've played 2 years of it non-stop all day every day. I probably stayed 10 minutes in character creation just because of the music alone (and trying to get my character looking exactly as it did back then - although I wasn't a shaman then). The whole intro experience was especially amazing for me since the Tauren starting zone was my first encounter with WoW in the EU beta, and a little bit of that spark did return. But is that all Classic has to offer? A trip back in time when we were wowed by the game for the very first time?
The folks over at Blizzard made a lot of dreams come true last summer when they formally announced World of Warcraft Classic during the Blizzcon 2018 keynote. The world’s most popular MMO has been chugging along for fifteen years now, receiving various upgrades and expansions. While the game is inarguably as good as it ever was, longtime players had been yearning for a return to the early days of the game and Blizzard seemingly agreed with that sentiment.