Personally, if there is any AD&D sacred cow left that needs to be killed off, it's the goddamned multiverse.
D&D has a lot of different, unique settings but the one thing that every single D&D setting has in common is that it isn't improved by chopping into pieces and throwing it into a mixing bowl with the pieces of every other D&D setting. To switch up the metaphor, when you pour all your settings into the kitchen sink, the only flavor you have left is dishwater.
If people want to mix and match all their different D&D settings, by all means have fun, but please stop asking the rest of us to accept it as the default.
Spelljammer is my favorite D&D setting, and it actually works better when you don't force it to accommodate all of the others.