I've played with M:TA, and found it intriguing, but it simply didn't have the sense of scale and grounding that arranging assassinations of the local lords did in Ars Magica. What I found interesting is that it allowed for even more possibility than Ars Magica, but at the same time they wanted to sell you zillions of faction and tradition books which really didn't add much. Again, it just doesn't compare to the excellent sense of scale that's possible in Ars Magica (and few other RPGs), because it allows you to dive right into medieval or any other pre-gunpowder history. You can kludge it into pretty much any other game you like, but Ars Magica is built for it, and it shows with elements like the covenant creation rules and such. Visiting Rome or southern France during the declaration of the Albigensian crusade is a great way to throw something completely different at a gaming group that thinks it has seen it all - big events happen where the players can't possibly deal with the entire thing from start to finish. This gives the player a unique freedom at the same time because it isn't strictly history, and you are decidedly not bound by 'normal' human limitations. (Unless you think we can all toss fireballs and lightning bolts from our fingertips.)
The idea of troupe play is still present, and although I don't remember the details too well, I think there's still some emphasis on having both a companion and a magus. Still, if you're going to write up every character that's likely to need a character sheet for a moderate covenant, chances are high that you'll end up with a pile of grogs so thick it outweighs the stack of magi and companion paper (if you did each on a separate sheet anyway). The number of grogs involved gets awe inspiring after a while; a small guard detachment, a kitchen staff, some local farmers, a leatherworker, maybe a metalsmith, probably several different kinds of craftspeope, definitely a mason or three and some carpenters, and all of a sudden you're dealing with enough people to form a respectably sized mob.
Another factor to consider is that character generation really is a lengthy process even once you've become familiar with it; there are no simple characters, unless you do something drastic like chopping the virtues & flaws system. Considering the vast number of extras needed, the group controller (on this matter) will end up either spending weeks drawing up characters and background, or starting a regular, "make a grog" period at the beginning of each gaming session. (This was my solution.)
On the idea of rotating storyguides - I found in practice that it often doesn't work too well, because in most groups there really is only one person (if that) capable of running a game in Mythic Europe without forgetting or overdoing either the mythic, or the Europe part. By all means farm out creation of NPCs, and encourage the players to think up the various things they'll want to do in establishing and protecting their covenant. Certainly my experience didn't run like a serial set of AD&D modules - aware players, intrigued by the scope to go visit the local fishing village, aren't going to settle for being railroaded through a typical dungeon-crawl "plot" more than once in a while.
One suggestion I'll make which I found very useful for bringing home the perspective idea (which isn't that easy because not many of us really understand the way politics works in any time period that well beyond the surface), is to as evil GM occasionally break out a set of character sheets for one of the groups they run into during the game. For instance, an older covenant of magi, a noble court, a group of scheming seelie, a dragons' moot... Mythic Europe is big, and there is no more effective way to make this understood than to have the players deal firsthand from the perspectives of other groups. Then roleplay them going through a covenant meeting or the like where they discuss standard business, or deal with a routine matter; it lessens the focus on the player covenant, but it really helps bring alive the immense interweaved events of a true world.
