The dream logic just doesn’t work. So you are on a balcony, sleeping and dreaming that you are in a hotel. Someone throws you over, your sleeping body is in free fall, accordingly the dream also loses gravity. It’s an old, clichéd idea, and untrue. I for one have had plenty of dreams interrupted, but not once has outside stuff managed to sneak in. But fine, the movie frankly comes out and says, look, this is how my internal logic works, ok? Sure, movie. I'm not the kind of guy who complains about warp-drives in Star Trek.
But suppose in that balcony dream, you have a dream within a dream, you dream you are on a beach. You get tossed, your real body experiences 0 g. This is transmitted to your top dream, and the hotel loses gravity. Now your sleeping dream body in the hotel also doesn’t experience gravity. This gets transmitted to the beach- But in Inception it doesn’t. Why? “Uh, let me get back to you on that later.”
Then, apparently they have control over dreams. One person builds them, one person populates them, and the others get to do minor things (conjure up pistols, change their appearance). Why not build cooperatively? Enemies chasing you, just conjure up rocks above them, or a great pit below them, done. Maybe the machine is designed somehow to prevent this, or more than one person mucking about confuses things. I dunno.
Also, in one scene, a character is shooting enemies with a rifle. Aemes comes over, exclaims that “you need to dream bigger!” and raises a grenade launcher. Wait, they can do that? Why not just dream up RPGs? Tanks? Helicopters? Lightsabers? Crazy futuristic space alien weapons?
Ok, so lightsabers would obviously tick the victim off. But so what? They can capture and torture him indefinitely. You could dream up some crazy torture equipment, I bet. Then there’s tanks. It should be trivial to dream up an invasion or military compound or something. They even do it! But curiously they won’t dream up tanks for themselves. I guess maybe it attracts the projections, but when you have an AC-130, who cares if their little snow-Humvee is attracted to you, honestly? Just gun them all down.
Then the machine. It looks like all it does is take venous blood, give it a little detour through the plastic hose where chemicals can be injected, and send it back in. The only connection is through the wrist. How does information get shared? What links the brains? Magic? To their credit, at least they didn’t actually go and make up some shared consciousness silliness. Oh well.
And the whole waking up business is fishy, anyway. Sleep, while somewhat mysterious, is very well known to be closely linked with firing of certain neurons and certain neurotransmitters. There’s no such thing as someone being in a sleep so deep they can’t wake up, you can always wake them up by injecting the right chemical. There’s comas and the like, of course, but a coma isn’t just a deep sleep. That’s stupid. In fact, given that what we see in the movie requires sedatives anyway, just cut the sedative. Soon the effect would wear off and the dream would destabilize anyway.