I find it quite interesting that you find MU*s more boring than MMO*s. The longest I played a graphical MMO was for 2 years, and I canceled three times for several months during those two years. The only thing that kept me that long was the fact that there was massive PvP on the scale of 100 v 100. It's hard to find that in a MU*. The gameplay, for most MMOs, is extremely simplistic and repetitive. Once you've mined ore, downed a raid boss, or used your same old boring heal spell for the 10,000th time, MMOs really offer very little else except for what the players might think up themselves. There are tons of graphical games where players have created more interesting content than the devs. The game turns into "Okay, let's take all our gear off and see how fast we can do this dungeon" or "Let's see what happens if we make the healer tank the whole dungeon." Talk about boredom!
In addition, I'm not sure if you're comparing the entire genre of graphical games (which includes more than just MMOs) to just two or three muds. What would be more fair is "How long did you last on LoTRO?" For me, I lasted approximately 1 month. Of course, I've lasted less than a week on some MU*s. Ultimately, I've spent more time in graphical games than on MU*s, but that's because I'm an admin on my preferred MU* and can't play it. Also, the MU* I actually spent a lot of time on shut down a few years ago and had a pwipe. Then they closed my favorite class.
Nowadays, I'm much more critical of the games I play, and I find a lot less holds my attention unless it's a game like Civ 4 or Harvest Moon. Graphics can't convince me to stay on a game since gameplay and storyline is what has always interested me in gaming. Most gaming systems, especially MMOs, don't take a lot of skill now, but they do take a lot of time. Some people, however, just love the graphics. Since I spend most of my time on MMOs playing a healer, I really don't see much more than health bars and the buttons that represent my healing spells. I find that more interesting than the graphics in most games anyway, but that's just me.
Lots of people play games and have no idea what they want from a game. Do you want progress? Do you want something pretty to look at? Do you want a ton of people to interact with? Do you want a good storyline? For everyone, a different combination of things appeals to them, but very few people sit down to figure out what they want. They have no idea why they enjoy the games they do, but you will see people slamming some game for not having some random feature that doesn't even fit into the game. A game shouldn't have EVERYTHING. It should have things that fit with the gameplay, but you see games try to do everything nowadays. They always end up with crappy systems that seem tacked on, and because players don't take the time to really understand what they like and hate in games, they will ASK for everything regardless of whether or not it fits into the world. There's a really popular game out there right now that added their crafting system on at the end without really thinking about it. The crafting system is simplistic. One click of a button, and you're done! It's only deceptively hard because of the mats (with VERY random drops) that several of the items require. How can you tell it's a bad system? The economy on EVERY one of their servers is completely borked. Except for a VERY few items, the crafted end-product sells for LESS than all the mats combined. Sometimes it sells for as much as 50% less. That's a borked up economy!
Anyway, I'm rambling. I don't think MU*s are any less or more boring than MMOs as a genre. SPECIFIC MU*s are possibly more boring than specific graphical games. The trick to ANY game is finding one that fits you personally.