I also have a hard time wrapping my head around it, but I don't think it's a 3D vs. 2D thing. I think it's a Tekken thing. In 2D games, you can generally block low and then react to overheads, and then you'll have a few universal system mechanics. In Tekken, you block high by default and react to lows. You can't crouch-block mids, but you can create a whiff punish opportunity by ducking highs or sidestepping vertical moves. Okay, those are some cool tools to use. Except some moves hit low that look mid. Some moves hit mid that look high. Some moves that look like they can be sidestepped will actually just track your movement. I desperately tried to find some rules of thumb that would help me actually play defense in this game, but the answer I kept getting was that I just had to know what those moves look like and memorize those properties for each one. That's not a skill I excel at, and it's not a hurdle I'm interested in grinding to get past, which is a bummer, because this is most I've ever understood or found a taste for Tekken.