as an aside, Mouse Guard is not "Burning Wheel Lite" - anyone who says it is hasn't played both.
The rules for MG are fairly streamlined, and narrative focused, but provide lots of crunch, too. And its optimized for very low to no prep.
It's VERY story focused, tho', and not aimed at physics enforcement. Then again, you're playing mice with swords.
Spirit of the Century
It's a hefty tome of several hundred pages. Once you have read and understand the rules, it plays as a no-prep game quite well. Experienced players can play it as a pick-up game, even - the majority of the crunch is in the stunts, which can stymie inexperienced player with their variety. Once PC's are generated, however, it's rock-n-roll time.
Only real hitch is that it's designed for use with 4dF, rather than standard dice markings. Plus side, once you learn one FATE system game, the others are all much easier to get to table.
Houses of the Blooded / Blood and Honor
These two are implementations of the same engine, hence listed together.
Prep will do you little good in these two - the resolution system pretty much ensures that nothing goes as originally planned, and the story twists along in ways that will surprise even the most machiavellian GM... because the twists and turns aren't his!
HOTB is a fantasy setting that's vaguely romanesque; B&H is Fantasy Japan.
Note that the system is: find your dice pool, as does anyone else with a stake, everyone sets aside some as "wagers", high roller determines success or failure and keeps all wagers; everyone else keeps half their wagers; each person with wagers remaining in turn says one additional thing about the action or situation, spending one wager; keep going until all wagers are spent. The GM always has a stake, if desired.
The majority of the game system is how to find those dice pool totals.
NPC's are often defined by players spending wagers to define things about them on the fly; one stat or statement per wager. (Which is how I wound up with a sophontophagic Jolly Green Giant as the PC's ally...)