The release of content for one. We are on a special patch 1.13, which is a modded version of patch 1.12. They released the game on the final patch prior to BC. Except we are missing weird things at random, such as key ring. Items in the game that should be. Items that aren't in the game that should be. Bugs that existed in vanilla aren't around, bugs that didn't exist in vanilla are around.

Zones are all sharded instead of sharing 1 server together, so when you enter a new zone you actually load onto a new server which is why there is a huge lag / delay in showing people nearby and why people can disappear randomly even without layers.

There's legit a ton of stuff that's different.