#1: What about the Sentinels?
What about them? Up until Grant Morrison's first arc on X-Men, the Sentinels were pretty much the most ineffective giant killer robots EVER. Outside of alternate future/reality stories (i.e. Days of Future Past), there had been more on-panel mutant deaths due to random hate mobs than due to these billion-dollar taxpayer-funded murder machines, and ever since Morrison, they STILL aren't very effective (i.e. Sentinel Squad O*N*E* - the Colonel Klink of the Marvel Universe). That's not a reputation I'd want to uphold, really. On the other hand, that's fairly well in keeping with how military contracts work in real life, where something is designed and promoted as incredibly badass while being anything but.
#2 Iron Man Variant Armors
I'll probably deal with these on a more case-by-case basis as time goes on, but so far all I really have to say about Iron Man's armor I've said here.
Short answer: He really couldn't. But certain parts of it aren't as insane as you might believe. If you believe that the material of the outer shell is a type of memory metal (similar to the memory cloth depicted in Batman Begins), then you could compact it fairly well. On the other hand, as Warren Ellis correctly pointed out in his Iron Man Extremis arc, the control systems (the sensors, the hydraulics and mechanisms that allow motion, the weapons, etc.) would be impossible to do in such a manner.
#4 Where's my Goddamn Flying Car?
Trust me, unless you want the sky to be filled with 300 mph fiery metal deathballs hurtling towards you, you really don't want this. As I've stated here (again). Short version: Cars don't generate lift, and you really don't want to trust the same people you don't trust on the highway with what amounts to small missiles, unless you want your hometown to start looking like Baghdad.