There are some gameplay issues with the WoW Layering tech (Layering is World of Warcraft's name for where you there are multiple shards for the same zone, and players may transfer between each layer as it balances. So instead of different servers, everyone is on one "server", and each zone is sharded to accomadate large populations). But the layering and single realm tech is just really well done. It scales to huge populations seamlessly. There is some polishing on zone boundaries and pvp, but the tech was pulled off without a hitch for a massive launch, millions of concurrent players.