My latest experiences in the WoW Classic Beta have made me more pessimistic about the success of the WoW Classic. To wit, the early zones, Elywnn Forest and Westfall, are already dead. Honestly, they died faster than many of the other "failed" MMOs which I've tried.
I ended up deleting all the characters I made and started fresh with a Priest. There are very few (like on the order or 5 or less) people in Elywnn Forest. Local chat is completely dead. I got up to Hogger, and then spent time on the weekend going, "LFG Hogger". After three different sessions, I finally got a level 9 dwarf hunter to take pity on me and we teamed up and killed Hogger.
Westfall is pretty much the same. There isn't even anyone just hanging out in Goldshire. In Retail, there's always people dueling or jumping around in Goldshire.
Classic is a game which requires other people. If there are no other people around, it becomes a very frustrating experience.
Now, maybe everyone is on higher level characters, and focusing on them. Though Stormwind was pretty empty as well. Maybe people aren't playing because it is Beta, and they're saving their powder for when Classic launches.
I guess my advice to people thinking about Classic is to make sure you do not miss the initial wave of players. If you fall behind, Classic will rapidly become a lot less fun.