HeroQuest has a great system, too. Robin Laws wrote it, so it isn't surprising that the system is great. It's built to model games in a mythic style, and scales well. The bit that I really like is that everything is integrated into one mechanic. All character abilities work the same way: strength, retainers, magic, equipment, personality, whatever. Whatever abilities are being used, the same kind of resolution works it out. In addition, good and bad effects of success and failure are integrated too - you can take 'damage' from failed contests in any area, not just in physical combat, and different kinds of damage work the same way. Lastly, there is a very nice way to model how multiple abilities can work together. The game doesn't have a set abilities list, just many suggestions, so each character can be (and, it seems after watching my group make up a hero band is) idiosyncratic and interesting.
There are two methods of task resolution, extended contests for things that are important to the story and simple contests for everything else. Simple contests are for generic encounters ("you meet a few bandits, everyone make a simple combat roll") rather than playing out in detail things that don't really matter. The other thing is that results are always worked out the same way - you get a level of success from critical failure to critical success and apply this to interpret the results of the contest. So a critical failure in a combat might be death, in a legal action you might be exiled from your community, in a seduction attempt you might be slapped and made a fool of. The lingering effects are based on this, so you can have 'wounds' that affect (say) anything that you need self-confidence for (in the seducation example).
Simple contests pit the hero's roll (d20, try to get under rating) against another roll (either your opponent or the world) and compare results to get level of success. Abilities have no upper limit, but abilities over 20 turn into abilties with mastery. So 21 is an ability of 1W (where W is the mastery rune) and 45 is 5W2. Masteries allow you to bump up your success level, so our chap with 5W2 has a hard roll - 1-5 on d20 - to succeed, but gets to bump up twice, meaning that the worst result you can get is a critical fail bumped up twice to minor defeat. Nice. Foes with mastery can bump you back down, of course, and you can spend hero points for a bump too (but these also are used for character development). The mastery mechanic is the one that is supposed to allow mythic level stuff going on without changing the rules, as people with masteries cancel each other out - running a contest between gods is the same as between peasants. Your peasant is in trouble if the god decides to stomp them, of course.
Extended contests are for stuff that's really important to the story and involve multiple rounds of bidding abstract 'advantage points' (AP), which the loser of each round will lose. When someone's out of AP, they have lost the contest and the amount they are below 0 gives you their level of failure. This system is pretty complicated to read, but looks like it might be okay once you've gone through it a few times. I'm reserving judgement on this - it looks like it could be very cool or a bit annoying, and only trying it will tell. In any case, it does appear that it will give you some cool back and forth contests sometimes, if one side loses AP then gains them back next round, which models changing fates in the contest.
I haven't run very many sessions yet but so far the game goes well. The characters tend to come out interesting and with plenty of motivation to drive their own stories. The system works smoothly andencourages creativity in how to solve problems (that is, characters using unorthodox methods - unusual abilities - to deal with things). One thing that hasn't come up is the use of extended contests - they just seem a bit complicated to throw into play when it's moving along fast. Simple contests seem to do the trick for the style of game that I run, so we haven't really felt the need to go for the more complex option.
The richness of Glorantha's background and history really helps too - I found the Heortling/Dragon Pass material has really given everyone a solid idea of the culture, the characters places in it and a good feel for how things work. This also feeds into the interesting characters.
Overall, a very good set of rules for running heroic fantasy stories and a very detailed setting with lots of cool stuff.
I'm pretty sure that the rules will easily convert to other games in a heroic style - I know I've seen plenty of discussion on The Forge about people using it for various other settings.