On the issue of guns and not picking up guns...
It varies, but military-grade weapons could probably "sit out" a while and still function, and function better than some of what they're carrying around. Pump shotguns are cool for scattering a room if you've got buckshot, and they're ok with slugs for putting people down fast. Good weapons to have, but with ammunition being limited then I'd pick up anything I could, to carry around.
On .50's, they're trasnportable if you have about two people. They've got enough people they could rely on to take one I think, that I'd take one. Men would be preferrable to do the job though, out of that group anyway. A .50 with a tripod is almost 130 lbs. I think. They're incredibly reliable though and I think even sitting out they'd be an ideal zombie mower if you could use them from a secured position.
I don't know about the reliability of an M16 that's sat out in the weather/elements for 6 months, but like I said, anything is better than nothing. You at least pick it up to clean later, get it functional... It'd be far superior to a lot of what they're toting in a situation where picking multiple targets quickly and firing at them is really important... not to mention you maybe can trade them too (The vato's mentioned roving gangs). I know some AK models can (in some cases anyway) shoot with sand poured through them, wet, etc. It's part of their popularity.
The current guy with the best weapon is the hillbilly (Merle's brother whose name I forget often). He's got unlimited ammo if he remembers to go back and pick up his arrow, and he's very skilled with it which isn't all that easy. It's quiet too, and he seems fairly fast with it. He's like the Snakeeyes/Stormshadow of Zombie death.
As far as driving a tank, Abrams tanks are gas hogs, but they're made fairly simple to drive, actually... if you could get one started and it had fuel, you'd have something nice to set up near a safe area... Unrealistic? Depends on who is in your group.
To me, it's not enough to detract too much from the show, but I would like to see them try to pick up some of the military gear and actually use it. Grenades, pistols/assault rifles/any small arms... If you were setting up shop at, say the sherriff's station from the beginning of the show, I'd want a couple of those .50's if you could get some folks together to haul them.
A Humvee would be a dandy ride too...
BTW, I don't look down on Shane either... desperate times make people do things purely selfish. Rick's the hero, and the focal character, but Shane is looking out for everyone too and himself... Being selfish in that situation would probably be common for many.
I too think they'll maybe go back to grab Jim. That seemed too easily set-up.
Will we ever see the hispanic family again? That was depressing.