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.
Nov 15 Classic is too easy for 2019 playerbase I'm not big on gameplay changes, really I'm not but... The game is just too easy for today's standards. I really think some of these raids and instances need a serious increase in difficulty. MC is a joke, most of the tier sets are unbalanced, and a lot of seasoned players agree but fear the toxic Classic community to speak up on this issue.Hopera237 Nov 15
So we asked ourselves, would it still be possible to deliver an authentic classic experience if we took our modern code, with all its back-end improvements and changes, and used it to process the Patch 1.12 game data? While that might seem counterintuitive, this would inherently include classic systems like skill ranks, old quests and terrain, talents, and so on, while later features like Transmog and Achievements would effectively not exist because they were entirely absent from the data. After weeks of R&D, experimentation, and prototyping, we were confident we could deliver the classic WoW content and gameplay without sacrificing the literally millions of hours put in to back-end development over the past 13 years. 

Nov 15 Loot sharing. Fools How stupid is the community about loot sharing? You know there is master loot right? Stop being dumb guys like holy crap. You also know that if you're the guy setting up a group or joining a group you have a option not to run or you can pick player's of different armor types... If you're a cloth wearer you just don't run or set up a group with other cloth wearers. Like this is something so simple and the community can't graps anything. I feel bad for blizzard sometimes with having to deal with brain power of this community. The community: "Oh, I didn't think about that. Derp, I'm an idiot."Plewtoe23 Nov 15

A lot has happened in World of Warcraft in the fifteen years since it originally launched, and over that time we’ve come to expect certain things, not just in WoW, but in games in general. As players explore Azeroth as it existed back in 2006 during the WoW Classic beta, they’re reporting bugs – but in many cases, these ‘bugs’ are really just features that are working as intended.

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