Coming as a newb to this board, but not to the anime/video game side of MUs, I'd say MU*es largely place RP first as their priority. Not going to go down the same points other people have made here, but I've seen some threads here talking about coded combat and the like, so here's my take on that instead.
I've been on some MUs where there's coded combat, but it's often with the idea that it's there as an optional tool/toy, specific to that MU*'s theme, to help with decisions, present ideas, and not overrule the flow of the scene. RP is a collaborative effort and consent-based.
One MU* I was on had status effects like Panic, which could be added to specific attacks. It was up to the player if they'd reflect this in their pose, and sometimes, it just wasn't a good idea to. Two other characters may be having a defining dramatic moment on that round, and drawing attention to my character would ruin that. Later on in another fight, however, if I'm hit with Panic again, I may use that as a prompt to show the effects on my character because it would sell some aspect of the other character who inflicted the Panic status. My character may even attack one of their friends, as long as I clear it through a page to other player of that character. The panic status is there in both cases, so the chances of my next attack hitting will suffer in the code, but I was in charge of how I expressed it.
That's there the difference is, for me. Code can be added and things can be automated, but if it gets in the way of telling a story, forces the theme to fit it (rather than the other way around), and/or doesn't help create opportunities for RP, it just doesn't work.
