The main thing I regret about MUs when I was younger was just getting started. If I could go back and not start, I would. It wasn't at all what I thought it was while I was playing it and it wasn't what I wanted it to be. I wanted an online version of a tabletop game (because my gaming group broke up) and it wasn't that at all. I wanted action and adventure and everyone around me wanted drama and TS. People would constantly look down on me and bad mouth me because what and how I wanted to play "wasn't the right way" (or some version of that). By the time I learned what was "socially acceptable" on a MU, I was so angry and resentful at having to play everyone else's way that I was just pissing people off and allowing people to piss me off on a constant basis. I never believed (and still don't) that MUing was a "collaborative writing experience". I'm not sure where that concept came from but I've rarely seen it apply to any MU*.
I was a very negative person for several years in large part due to my frustrations on MUs and with life. Eventually, it made me hate online gaming altogether. Self sabotage is a bitch like that cause I was basically addicted like a crackhead. I loved the high of MUs but couldn't see that the fun times were increasingly outweighed by the frustrations and shit slinging I was taking my way. Whenever a new game would pop up and I'd get that little spark of hope that this one would be better and I'd be let down all over again. I've spent hours, days, weeks, months, years of my life online on a MU, trying to get 'fun' to happen when I could have been out actually doing fun stuff.
It wasn't until I was older and wiser that I realized that the trade offs were not worth it and I was happier not playing at all, than finding little bright spots here and there while being frustrated and annoyed (or worse) all over the place more often than not.
I still think the biggest problem in MUs is the toxic community of players that doesn't really change. Not a lot of new MUers. Same bad people, only a few of which grow and mature. Others carry grudges for years or decades.
I still think too many people bring their RL issues (mental or emotional) to games and expect other people to accommodate them. MUs are not group therapy sessions or outlets for you to unload on others.
I used to think it was fun to find new and unique ways to "win". I swear I learned that from reading "Ender's Game" when I was little. But I missed the part where everyone fucking hated Ender and wanted to murder him. So I don't think its applicable on MUs anymore, if only because it annoys everyone else and ultimately makes everything else less fun for you in the long run.
I still think ICA has to equal ICC if you expect to have any theme on a MU. And not just between PCs, but from staff too. They need to bring the NPC folks that keep people in check in the first place, not just the NPCs that end up as event fodder. Every MU I've been to people do eyerolly stuff consistently because there are no real consequences. People are far too touchy about character death and character control and so consequences go out the window and inanity builds more and more until it overflows into complete theme abandonment and obliteration and even the staff can no longer rein it in.
I have learned to temper my expectations on what I'm getting out of a hobby (not just MUs but any hobby) and that has helped me enjoy the things I do in my life a lot more. And to better evaluate if it is something I want to keep doing or when to quit. I've learned that I'm not responsible for anyone else's fun so if I want to quit a game I can do so and not feel guilty for the people left behind.
I've learned that good staff are undervalued and underappreciated. I used to believe that anyone could be a good staffer so long as they had the right mindset of what the game is about. I've since learned that such is not true.
I've learned that bad staff are overly tolerated. I used to think you can just ignore bad staffers, but I've learned that just their presence infects the game and spreads negativity which spoils a game from the inside out.
And I've learned that no matter how right you are or how great your idea is, it is far more important how you express those ideas or opinions that determines whether people will take you seriously. It applies to RL too, but far, far more on MUs.