I've only returned to one MUSH I've left. Ever. I was part of the very first story arc on OtherSpace and we all did some fine roleplaying. Now the downside to the story arc school of MUSHdom is that one does tend to feel the weight of the plot careening about on it's own inertia. But that's simply the price of a world that's being driven /somewhere/ as opposed to idling nowhere in particular.
After taking a couple years off I found myself back and picked up with a new character - a little wiser for my previous experiences. Amazingly almost all the original players are still around as are many of the same characters. There are stories, logs, detailed information and much more on every aspect of OtherSpace. And the buck stops with Wes Platt aka Brody. He invented the setting and pulled together an awesome team of admins and roleplayers that's constantly growing. This is his baby but he's willing to let others contribute to the setting and storytelling. Indeed, the arcs themselves appear more amenable to player interference. Frankly, I don't know where he finds the reserves of energy to keep up with it all.
Me? I've got a new character, a passel of logs I'm quite proud of and I'm developing a tiny part of the world which my character belongs to. This is some fine old time roleplaying. Just be ready to let go and roll with events. That's the key to enjoying them and OtherSpace as a whole. It ain't hard. :)