I have to admit that I wasn't expecting Blizzard to actually make WoW Classic. It's basically a second game at this point, and I didn't think they'd be willing to go that far. But Blizzard is going to try, so we'll see how it works out.
I would like WoW Classic to be a success. If it is successful, maybe Blizzard will start nudging normal WoW back towards Classic, adding back some of the small decisions, inconveniences, and restrictions that have been smoothed away. Perhaps they will also dial back the focus on transient gameplay, bringing back an unapologetic focus on "extended" content.
Those are all changes which I think have weakened modern WoW, and I would be happy to see the pendulum swing back in the other direction. Of course, the opposite might happen. Blizzard might leave WoW Classic for people who like the rough edges, and make normal WoW even more smooth, in order to differentiate the two.
That being said, I think that a lot of people will try WoW Classic, run up against those changes, and start complaining. Are people really going to enjoy reagents, ammo, shards, dead zones, requiring exactly 40 people in a raid, top guilds poaching players from lower guilds, the hybrid tax, hybrids forced to heal, few viable specializations, very little loot, and all other sundry rough edges which have been smoothed away?
My prediction is that a massive number of players will sign up for WoW Classic, but they will all dwindle away in a year or two. The players remaining will be complaining that their guilds cannot maintain raids, and calling on Blizzard to smooth away the restrictions to attract more players.
I also think the mod community will make a determined effort to eliminate many inconveniences via mods. For example, I bet we see a Party Finder mod (with Gearscore!) quite quickly after WoW Classic is released.
Finally, I think the community will be surprised at how easy WoW Classic will be. I don't think WoW Classic will prove difficult for raiders used to modern mechanics and theorycrafting. Especially as everyone knows the strategies for all the old content.