Sure, they are diffent like that, but at the same time they are easy ways of bypassing MR. And that has been the core of the discussion...
A hard, two-handed blow with a pole axe is another way of bypassing MR. Or a cream pie in the face. Or a swift kick in the crotch. Are those, then, the same as the magical mud scenario? 
The core of the discussion is not "easy ways to beat MR", ulf, but easy, magical ways to do so, and whether those are a good thing, or not.
There is nothing magical about the falling boulder, ulf. We are talking about magical effects, because only magical effects interact with Magic Resistance. (Starting to see the theme here? Hint: Starts with an "m".)
Get some coffee, and come on back once it kicks in. 
Mud? 
Anyway, remember that while you're not hurt by a falling magical bridge, you will be hurt by falling onto that same bridge. So the issue of magical water isn't clear cut: you surely will not drown, but you may very well suffocate.
Not to shoot myself in the foot, but here's a different situation that I just thought of...
MuTe 10 - change 10 cubic paces of dirt into a cubic inch of gemstone, duration Concentration. (10 of dirt => 1 cu p. of stone = 1 cu ft. of metal = 1 cu in of gems.)
(Base 3, Touch +1, Conc +1, +1 gemstone)
You get the giant magical frog to eat the metal cube- maybe hide it in a giant magical fly, dunno, whatever. When the spell duration ends... what happens?
Sounds to me like the frog dies, or at the very least is not happy.
Don't know how I'd work that- I think I'd have to allow it. The only question is- can a magical gemstone be eaten (voluntarily, if unknown) by a creature with MR, and I'd have to say... of course!
Hmmm...
Well, at least an archmage isn't likely to fall for it.
The frog would only eat it if it penetrates. If it does... 
On a related note, we had a great time recently as a couple of grogs. Us grogs discovered that a particular mat had been lain out to assist with the teleportation of the Primus of house Bonisagus. We really tried to have a chicken hanging there when she arrived with her entourage, but we got caught and our plan foiled. 
Last I checked there isn't anything magical about gravity either... And it's gravity that makes one fall once the surface one stands on no longer can bear ones weight. The need for air to breathe isn't magical either, nor is the ground once the spell ends...
All in all, most of the things we've been talking about have had the magical effect leave them before the effect reached MR. Much the same as the ReTe effect will have left the rock.
Mostly, the only reason to disallow this manner of affecting people seemed to be game balance.
Good one, this isn't very different from a spell to expulse all air from a zone (aside from the fact it's more difficult to come out of the zone)
Except that "gravity" doesn't exist in Mythic Europe.
Again, you're applying modern paradigms of physics to magic, which, imo, is a big mistake. But whatever.
Good gaming.
Not only it is a mistake, but it changes all the themes and tastes of an Ars Magica game. The book specifies that the only natural laws currently working in this world are the medieval laws in the sense of the laws people thought they were applying. (exemple with the diseases, that don't spread by physical contact because they are only inbalances of humours)
Hold on you two 
The Aristotelian concept of why things fall down wasn't called "gravity" as I recall, but that's the word we use for that phenomenon involving weight. It's the same spectacle with of course different reasoning, and it's natural not magical. IN Mythic Europe things fall not because of gravity as Newton described, but due to natural forces involving weight very much like gravity. What is that phenomenon called anyway?
Things suffocate naturally and things fall naturally. Can a magical substance cause a natural effect in order to bypass MR? Maybe.
-m
I know (because I read it as a comparative translation from the original) that the effect of an inclined plane or ramp, as an aid to moving heavy objects to a higher level, is explained because the ramp... (wait for it!)... actually makes the object lighter.
Extrapolate that one back to a concept of gravity, and get back to me.
Newton didn't come along until much later. Before then, the effect we call Gravity was really not a concept, it just wasn't part of anything people thought about, no more than an accurate solar system model, spiral storm systems or oxidation to produce fire or rust. Handy if they knew it, but until someone thought of it, no one missed it.
People thought about those concepts, they just thought differently than we do now. The trick of this game (and the fun of it ) is to try to get historical accuracy into those concepts. In 800 years everything we know as truth wil probably seem just as goofy.
Things fall because it's in their Essential Nature to do so, not because of some external "force". That's why if you fall onto a magical bridge, your Parma doesn't shield you from the impact - it is the fall itself that caused the damage, not the impact at the end. You will be exerting a force on the bridge, not the other way round - action does not equal reaction.
And birds can fly because that too is in their Essential Nature. 
And yet, a common one.
Heh, as if! What a quaint and naive notion.
Why, if that were true, then when a horse jumped, and equal weight of dirt would have to be moved down- and we all know that doesn't happen.
Worse, it's as if the flight of angel's wings need push on the air or something.
Really, a bit of basic logic needs to be applied. Philistines. 
By gravity I refered to the natural tendency everything has of moving down. (which was known during the middle ages, if not by that name)
The modern consept of gravity (the tendancy for things to be drawn to each other), is something completely different...
Yes. I think Aristotle explained the phenomenon as something having to do with moving towards a proper place among the celestial spheres in proportion to weight. We call that "gravity," and things fell naturally (for whatever reason) in the medieval period as well.
Things fell, and things suffocated naturally. It's not about modern paradigms--it's linguistic shorthand. It doesn't erode his post (unless you're arguing that things didn't naturally fall in the medieval paradigm). This point is jumped on to avoid a more relevant issue (which is fine, but it's clear).
Spoilsport.
As I said before, with Parma you will not drown (get water in your lungs), but you will not be saved from the lack of air.
Unfortunately, I think you may be right, and I don't like it (not that I don't like you being right--that's fine--I just don't like that answer in the same way I don't like the issue of pink dots or someone "missing" just three people out of a collective mob that is hit with a targeting roll involving a pit larger than a church).
This isn't quite right. Things don't "fall down", they move toward the greatest concentration of their essential nature. Rocks fall, because the earth is the center of the essential form of earth. Water flows toward water. Fire "rises" because the greatest concentration of pure fire is above the earth, in the sphere of fire. Humans are a mix of the four elements. So are all the other elements, for that matter. Plato drew a distinction between the elements that exist in our reality and the true form of the pure element. So water in a cup is still a mix of water, earth, fire, and air. It is mostly water. Humans, being mostly earth, "fall down" because the "earth" in them is pulled toward its natural home, the sphere of earth. It's a fine point, granted.
The English word "gravity" was first used in the 16th century to convey the meaning we currently give it. Prior to that, in and 13th century Europe, the Latin "gravitas" meant serious or dignified. This is neither here nor there, I guess. No medieval would use the word "gravitas" to mean "falling down", but they most certainly were aware that if they dropped something it fell.
This doesn't advance either side of the argument, I know.
I like exploring medieval concepts and enjoy reading medieval philosophy. I've found it's hard to excite players about the celestial spheres (for example), Aristotle's fifth element, or the three kinds of lightning (produced by invisible sheets of ice floating in the sky). When we get to the magic realm and magical creatures, all bets are off. In my opinion, magical creatures are immune to the (medieval) laws of physics. Do minotaurs need air to breathe? Why? Do they copulate? Produce offspring? Are they a race or an individual? Answering "yes" to those questions makes the minotaur a D&D minotaur. It's amazing, by the way, how fertile the D&D universe is and how many types of species can cross-breed with each other. Answering "no" makes it an individual beast of legend, which (to me) is all Ars.
I think the point is, will you, as a storyguide, allow a level 15 spell to kill a Might 20+ beast. I won't. I'd award the player by allowing the spell to slow down the minotaur, give her and her pals a bit of breathing space, but that's it.
(Mario and I play in the same saga, which is why I'm responding.)
Matt Ryan
That's the reason I hang out on theses forums, I always learn interesting things. 
Well that pretty much close the argument I guess (well at least for the minotaur).
I agree it a choice, no "right answer" just use whatever sounds right to you !