I've seen a couple of different weapon chests. Two from the original game, one that looks like a car trunk, etc. Do any of these containers have an higher chance of containing higher level items? Do their contents scale with your level? Do all players see the same loot?
Yes, different items have different loot. Most have "minor loot" (ammo, cash or health) but certain containers always have guns. Containers that always have guns also tend to have better weapons on average.
Chest contents generally don't scale with your level (though in Borderlands 2 they did in playthrough 2.5), however the Golden Chest items do scale with your level
Stuff that has guns
These chests always have guns and the items they drop are noticeably better than the typical gun drops from baddies or "non-gun" containers
- White gun chests: white with two sliding doors on the top, holds 4 pistols, 2 mid size guns or 1 rocket launcher. You'll often get a mix of white and maybe green guns in here.
- Red Chests, the oval shaped chests which hold ammo in addition to the same amount of guns as plain gun chests. These ones have rarer loot just like they did in Borderlands 1. They almost always have greens, often a blue or even purple/orange loot.
- An ornate gun chest which seems to be at about the same rarity as Red Chest items or better
- Outhouses which, surprisingly, usually hold two midsize/large weapons of good rarity; better than plain chests. The toilet also erupts with other...goodies. Usually minor loot.
- Green Dahl chests: These appear to be maybe slightly better than White gun chests, but not much. The guns aren't all made by Dahl, either.
- Red Dahl chests: again, slightly better than average but not amazing. These chests hold a couple more guns than White/Red chests. Again, any manufacturer's guns can be in here.
- Golden Chest: A special chest in Sanctuary that always has Purple or Orange guns. You need a golden key to open it. These guns scale with your level so open it as late as possible to get the best guns possible!
Stuff that has minor loot
- Most other containers have a small chance of having a gun (usually of normal rarity, sometimes better), but higher chances of having money or ammo, or less commonly health. Most containers fall into this category and the items they have are very random but usually minor.
- Tackleboxes (or something shaped sort of like one) always have money and nothing else, just like Borderlands 1
- A green ammo chest always has a large array of ammo in it and will never have money or guns. I'm not familiar with any other ammo-only chests; these are identical to the Borderlands 1 ammo chest.
I'll try and get pics on these while I'm playing
All players in coop see the same loot except when it's mission loot (you get slightly different reward items).
With regards to containers that can only hold certain items:
- Mailboxes contain either ammo or Eridium (I can't recall ever getting an item or health from one)
- Narrow tall metal boxes (can't describe them very well), which only contain either money or health (they appear in Bloodshot Stronghold)
You'll also see a bunch of exploding items (gas tanks, gas canisters) which have a green light like unopened chests. If you blow these up, they drop ammo and money, and (possibly, I can't remember) a common item.