Lot's of weapons can be scavenged as you go. I simply saw no need to waste time shopping and spending precious mil spec ammo that makes any gun that uses it quite powerful anyway. Sure they put lots more such ammo in the areas at the end, but having a stockpile of it avoids searching for caches whilst you're being attacked by waves of Nosalis. The first place it's very useful is in Biomass, and there's no decent caches of mil spec ammo before that point, just a few rounds here and there.
On the AI, I was referring to the creatures only. Once a fight breaks out with humans, you'd better be prepared, but the creatures were too easy, poorly animated, and dumb as bricks. Would have been nice if they'd given them some clever intelligence like the Aliens in AvP 2010, whom like to hide in the shadows.
Rage's AI made sense because they were crazy mutants and could pull off acrobatic, wall climbing moves. Metro's AI is more straight up military, where it would make no sense. The human AI was plenty good enough, the creature AI though was lacking. I don't expect them to make Nosalis climb walls, just make them more clever at hiding and flanking. As is they act invincible and all you need do is find a choke point and blast them, or in some cases like the tunnel you seal off, run past them.
Another complaint I forgot about, is some creatures, like the big ones in the library that are harder to kill, are too easy to sneak past. Most would choose to sneak past and save health and ammo I think. For all the potential this game had to scare the crap out of you, Dead Space 2 actually does a better job of it, and much of it is due to superior creature abilities. I also think Dead Space 2 does a much better job of building atmosphere and suspense.