I know it’s not iron:), but surely not made of anything in the real world. It’s not supposed to derive its strength from the chestal power unit is it? And what property inside the suit keeps Stark from being injured by impacts? I’m thinking primarily of the impact with the fighter jet in IM1. The wing gets torn off the plane and IM jokes about it. Here’s what gets me- the impact with the jet would be, imo, magnitudes more than impact with the Earth at maximum falling speed, yet falling to Earth is a scenario present in both IM1 and 2. In 2, Hulk dramatically catches him, but IM should be able to just shrug it off.
From the first film:
[QUOTE=Tony Stark]
Iron Man. That’s kind of catchy. It’s got a nice ring to it. I mean it’s not technically accurate. The suit’s a gold titanium alloy, but it’s kind of provocative, the imagery anyway.
[/QUOTE]
(source: Iron Man (2008) - Quotes - IMDb).
Not that the explanation helps assuage any of your consistency or plausibility concerns.
At least in the comic books, IIRC, the strength of the armor comes not just from the metal itself, but also from a reinforcing energy field.
“Chestal Power”. 
And, here’s an awesome bit of technobabble from the Marvel Wiki entry on Iron Man:
Are we talking about Pepper’s suit now?
The suit can…but how does it keep the occupant’s insides from sloshing into pink goo, or his brains from squirting out of his eye sockets?
Inertial dampeners.
ETA: which are made of Notreallyanelementium.
The suit is filled with packing peanuts.
Oh come on, you can do better than that:
The suit is filled with packing peanuts made of nano-foam which distributes any shock directly into the sub-atomic structure of the Universe using quantum particles before it can affect the wearer of the suit. It should be noted that this distribution of energy tends to cause greater damage to the impacted surface than would otherwise have occurred.
That’s how you fan-wank.
Obviously, it’s through the use of the unstable molecular fabric created by Reed Richards for the Fantastic Four, and shared with Tony since they’re on the same side. This is the stuff that doesn’t burn up when the Human Torch flames on, becomes invisible when the Invisible Woman vanishes, stretches with Mr. Fantastic, and, uh, covers Ben Grimm’s nether region.
True.
But you have to admit, that’s just a lot of fancy verbiage that boils down to “Inertial dampeners made of Notreallyanelementium.”
Others have given the answers posed in the title, but I’ll take a crack at this seeming inconsistency. Assuming the suit itself can withstand the shock of hitting the ground or being hit by a plane, then it’s just a matter of countering the impulse of the impact. In the case of the plane, he’s aware and his suit is powered, so perhaps Jarvis has some kind of countermeasure to handle a predictable impact. Thus, as long as the suit can precisely time thrusters in the opposite direction and the suit can withstand the high G-load, then the overall momentum of Tony Stark’s body doesn’t change significantly and he can take that hit with minimal stress. Or, perhaps the repulsor technology also acts as some sort of inertial dampening field, but that seems to be a little out of the realm of the universe that they’ve been working in.
Regardless, either of those explanations would also cover why he needed to be caught before hitting the ground in The Avengers, because Tony had lost consciousness and the suit had lost power. Thus, whether it was some kind of counter impulse thrust or inertial dampeners, the suit wouldn’t be able to soften the blow and he would have been killed instantly upon hitting the ground had he not been caught.
But as everyone knows, fancy verbiage is a necessary component of proper fan-wanking. Really, it’s like I’m dealing with noobs in here…
Not a noob-just someone who’s been around long enough to know that in comics, nothing is canon. There is absolutely nothing a writer can put to paper that the next writer cannot undo.
Yeah, for some reason the smiley got chopped off that post. Weird.
But what you’re referring to here is retconning! An entirely different issue with its own verbiage requirements (usually involving some sort of multi-dimension/universe/evil twin sort of thingie).
Tony is really possessed by the Phoenix Force, so simple things like inertia don’t effect him. This will come out whenever they need to replace RDJr. in the movies.
High forces or accelerations aren’t intrinsically harmful to the human body. The problem is discrepancies of acceleration. If some parts of you accelerate at different rates than other parts of you, those parts are going to separate. The solution, then, is to apply the appropriate force to all parts of the body at once. This is apparently something that Stark-technology repulsor beams are capable of.
I think you all are going about this the wrong way. Think about Tony Stark…brilliant, charming, rogueish, but flighty and unpredictable, has a thing for redheads. Uses technology that doesn’t make the slightest bit of sense. Anyone else we know like that?
Sometimes wears a bow-tie.
It’s obvious that Iron Man is actually Dr. Who. His suit is just a redesigned TARDIS.
Roger Moore is still playing James Bond?