I've always liked, and wanted to see coded, a variation on the Chaosium CoC system. When you successfully use a skill during play, it gets flagged. And at the end of the session or adventure, you roll your flagged skills, and if you /fail/ that roll, you gain a couple of points in that skill. It creates a curve where skills you're good at are easy to flag, but progressively difficult to improve (because the better you are, the harder it is to fail). In a MU* setting, I suspect it'd be something like 'successfully checked skills get flagged (automatically or manually), and at the end of the week, you may choose to execute a command like +improve, that automatically rolls every flagged skill, tells you which ones improved, and by how much. Now, this works better with percentile systems - I note that the new version of UA seems to be leaning towards this, as well.
I hate automatic XP, though. I like XP from dramatic failure, aspirations, breaking points, and negative Conditions - those are fun, and incentivize play in interesting ways. I like vote for selfish reasons - mostly that I like to vote for people, but I do /not/ want votes that they automatically know they receive, because that leads to that bullshit of <OOC> Player says, "Voting for you all, now." and then obviously waiting to receive notices of return +votes. I also liked Darkwater's little message where you could show your appreciation with puppies and all sorts of silly things - no XP, but it still incentivized showing people hey, you were awesome.