I actually *wasn't* particularly interested in WoW Classic when first announced; I figured I would probably log in for a few minutes, have a little nostalgia trip, and then never play it again. However, when playing the "demo" during Blizzcon, I found myself getting surprisingly excited about it. I had forgotten just how many little things were different, and it felt like a much more "grounded" experience than even the current WoW (which increasingly seems to parallel shitty mobile games in design).
I also walked away with some major concerns, though.
Chief among them; phasing. As cool as the concept seemed during Wrath of the Lich King, it's been made abundantly clear that it's been detrimental to the game. Temporary "cut-scenes" are one thing, but when I recently tried to level a Dark Iron with my father, we found ourselves barely able to even play together due to the insane amount of phasing that happens.
And I consider "sharding" to be an extension of that, which is obviously the real concern. I can't speak for anyone else, but I WANT to log into a laggy mess because of too many players. I WANT to wait in line to kill quest mobs with other players. That is quite literally what the Classic experience was. That's how you socialized with players. The idea that Blizzard is going to artificially segment players in WoW Classic -- even just temporarily -- is completely missing the point, and I worry may represent a fundamental misunderstanding of why players are demanding a faithful experience.
Beyond that, I've also been quite shocked how much has come out about Blizzard not even having access to previous builds of the game. Like... what? Which, it's great the game will run as smoothly as we've come to expect, and obviously hardware has evolve in the many years since WoW launched. But there was still something unsettling about mousing-over gear, and instantly seeing a pop-up window showing the change in stats. "That wasn't in the game back then", I thought, which made me wonder how many other things had been modified. It made the experience seem a touch less "authentic".
All that said, though, I'm still pretty eager to play it. Again, provided it feels as authentic as possible (as opposed to "as authentic as we felt was best"). The current state of WoW, both in gameplay and storytelling, has completely killed my love of the series, and I'm hoping that WoW Classic can rekindle that flame. I've even heard people suggest that "maybe Blizzard could springboard off of it, and create an alternate continuation for WoW". Which, there's no way in hell that will actually come to pass, but... well, the idea of a truly deep RPG WoW, taking the story in a different, better direction, is certainly appealing (ie. never making Kael'thas or Illidan evil, maybe Arthas and Ner'zhul aligning with us against the Burning Legion, etc).
TL;DR
I have some serious concerns about WoW Classic, but I think as long as they don't implement any phasing/sharding stuff, and keep it as faithful as possible (for better or worse, barring obvious bugs), I think it may actually be a better game that current WoW.