I received a beta invite for Classic, and had also been an alpha and beta tester for the original game 15 years ago. That isn't unique; plenty of people in Northshire Valley had had the same experience, suggesting to me that active players who were Day 1 WoW gamers or had participated in the original tests might have gotten priority invitations to this one.
Things like Tauren melee reach being bigger, which is something I experienced going away live during Black Temple Illidan progression (at least for that particular encounter), is confusing to players as it's a significant advantage over other races in some instances. No quest tracking is another apparently confusing issue, as many Mankirk's WIfe seekers well know, and there's a whole lot more where that came from:
The closed beta for Blizzard’s upcoming World of Warcraft vanilla server option launched on May 15th, and Blizzard has now listed some of the apparent issues that the development team is aware of, including class and combat related issues alongside missing textures and more. Blizzard mentions that this list is not a complete depiction of what they’re aware of, but a sampling of issues they believe players may encounter on regular basis.
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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.
The original World of Warcraft was released in 2004, and World of Warcraft Classic aims to recreate the state of the MMORPG before its first expansion, The Burning Crusade, was rolled out in January 2007. Previously, players looking for a “vanilla” version of the game were only able to experience it on custom servers such as Nostalrius, which Blizzard shut down in 2016 to protect its intellectual property.
I recreated my very first character -- a human warrior, because in the last-push alpha test I joined in 2004, there was no Horde -- and logged in. Immediately, I was surprised by how good the graphics actually looked, for being 15 year old textures-on-polygons. Warcraft's bright colors and cartoony aesthetic continue to this day, so all the increased resolution and better-contoured characters in Lordaeron don't really change the game's visual aesthetic.