If anyone has played tabletop RPGs, you’ll be familiar with the idea of using the same core mechanics for multiple different campaigns.
Dungeons and Dragons in particular is well known for the wildly different settings (including Planescape, Dragonlance, Forgotten Realms, and loads of others) and that’s before you even start on individual GM’s homegrown settings. These are all, incidentally, settings with radically different geography, histories, races, and can vary widely in genre and theme. (eg. Ravenloft is a gothic horror setting, Spelljammer is about space pirates on flying magical ships.)But they are all based on the same core rules.
Part of the appeal of an expansion or DLC to an existing computer game is having an expanded setting with the same core mechanics. Probably even the same core player character.
Part of the appeal of playing a game in the same genre as an existing one is having a new setting with similar core mechanics, but with a different player character. And maybe enough tweaks to add a learning curve. OK, this doesn’t explain why Blizzard like to tweak their mechanics for each expansion, but you can assume the gameplay experience will be familiar enough to not put people off.
So I never quite understand why people complain about ‘WoW Clones’. Do players who like FPS complain about CoD clones (or whatever the ur-game is for FPS)? By all means complain if the game isn’t fun, or the balance is off. But if you’re bored of the core mechanic, don’t know what you want, or just feel burned out in general, then play something else. It is entirely possible for a game to use typical MMO core mechanics (and we probably wouldn’t even be able to agree what those were) and still provide a breath of fresh air in other respects, to people who don’t actively hate that gameplay.
In some ways, I wish one of the big MMO companies would find a way to open source their mechanics and let other developers work on a wide range of different expansions, so that players could take their favourite character into multiple different games. Just like the D&D campaigns.