It would be interesting to see someone do a software usability study on MMORPGs for new users. Coming from City of Heroes to World of Warcraft is quite a shock, given how casual-friendly CoH is. Anthony stated that WoW was actually a step up in usability for him, however, since his previous MMO was Final Fantasy XI which was very complicated.
I imagine, since most games are rather different, it would be hard to compare them to each other, but the elements they share in common could be tested, in theory. I imagine the most useful single number to boil down would be gameplay hours until decent proficiency, but since you can't directly define "proficiency" across games, it would be almost impossible to get such a number.
Mokalus of Borg
PS - Personally, I think City of Heroes has a much nicer UI than WoW.
PPS - It's more helpful but still isn't intrusive.