My first foray into anything MU- was when I was 16. I found a place called TigerMUCK which was, yes, a furry MUCK, but the owner - someone who is probably one of the most decent people I've ever met- had a very strict policy of enforcing a PG rating and no fooling around anywhere near minors. He was pretty uncompromising about that, too, which resulted in a rather very friendly and non-threatening environment. This didn't exactly prepare me for what other furry places were like- I decided to look at the other MU*s and went straight for FurryMUCK under the assumption that bigger was better.
It went about as well as it could be expected:

... long story short, a few years later I decided to venture out of TigerMUCK again, this time for Discworld MUD, and I absolutely loved it. Having my first taste of full-on RP, I moved quickly to other MU*s- there was an X-Men one I can't really remember the name of, and a few others- but it didn't really last too long. I had to move to another country (this one) to pursue my degree in music, and for many years my social life and past-times were very thin on the ground between rehearsals, classes, lessons, practice room hours, more rehearsals and trying to get a handle on this college thing everybody was talking about.
I didn't stop roleplaying at the time, though, but the pace of RP that I needed was a much slower one than the real-time MU*. So I joined the Napping Cat's Dream forum. The roleplay spun out of the Master Zen Dao Meow webcomic, which was a rather whimsical and trippy comic dealing with eastern philosophy. The place lasted for ten years, outliving the comic by several of those, but it was neat because the comic creator/artist and the admins (who incarnated characters from the comic) would regularly engage the forum members in roleplay and discussions, and members who contributed to the community/rp/discussions in significant ways would eventually be deemed 'psychedelic avatars' and receive avatar cards of their characters, fully illustrated by the comic artists. It was a rather homey place for many of us, and when it closed its doors in 2009 many of us felt a little homeless, RP-wise.
After I graduated and started building up experience before a Masters degree, I got sucked into Second Life- mostly because of its music community at the time. I toodled around SL for years doing the concert thing and hanging out with Mel Cheeky and Frogg and Jaycatt and co, and then I got recruited to be a faction leader for a roleplay sim there. A Final Fantasy VII one. I even did the special effects for their combat system. But after two years of that, I decided to call it quits. It would be a few years later, when I started my Masters degree, that I decided to poke around mudconnector and look for some MUs- and that's how I stumbled upon HeroMU* and, later, CoMUX.
So, long story short...