It'll mean you'll often get attacked, which can be really frustrating when you just want a relaxing evening of questing.
One of the biggest differences to consider with WoW Classic is that the servers are structured differently. In Classic, there are PvP and PvE servers. Once you start a character on one of them, they'll be stuck there unless you pay for a character transfer. Consider carefully whether you want to get involved in open-world PvP before playing on a PvP server. It'll mean you'll often get attacked, which can be really frustrating when you just want a relaxing evening of questing. On the other hand, that extra element of danger can make the world more fun and offers more opportunities for social interaction with other players.