In a sense they’ve retroactively claimed all existing mods and maps as theirs. (All your existing custom maps are dead in the water with no way to replay them, and because of the new EULA, Blizzard has claimed ownership of any customs maps, campaigns, and game modes that might be converted to the new client anyway).
Here’s just a few things that are wrong with the game, starting with the big one: I don’t own Reforged, but the people who do summed it up this way on Reddit:
Is the issue that folks with Classic keys cannot play any longer without purchasing the new version? Why do people want the classic version? The Reforged version is just a prettier version of the original, including the same cut scenes (and animations) everyone seemed keen to see improved.