I don't even remember the name of the server or software, but even back when WoW was contemporary I had a lot more fun playing on free servers with extended XP, even though pretty often bosses would be buggy or not quite the same as in the real game. It was so much more playable and casual compared to the early WoW (or worse, EverQuest which came before). It's a shame game companies can't find a way to embrace or even profit from these kinds of servers.

MaNGOS, ARCEMU are the two that come to mind. I was really young and it took me so long to figure out how to use svn to get the code and then to compile it, so i always used repacks. Some guy on MMOWNED helped teach me all this stuff, he still makes private server content today.

Their business model is to lock you into a monthslong or yearslong grind, unfortunately.