Borderlands – Why can’t I loot Dr. Mercy’s body

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).

Oddities

• The Golden Chest requires a golden key to open and always has purple or magenta (e-tech) items in it.
• Dice Chests from Tiny Tina's Assault on Dragon Keep (AKA DLC4) let you roll one or two dice and the higher the roll the better the item.

To my knowledge only white loot despawns (this includes things like ammunition) and while I don't know how long it takes specifically it is possible to tell by looking at it. The item steadily shrinks in size until it basically disappears. So yes, it does seem to be based on time.

I'm fairly sure items rarer than white (green, blue, purple, E-tech, orange, obviously) don't despawn. I've left them lying in open chests before and come back ten, 15 minutes later and still seen them lying there. I'm not certain about them being on the ground though, if you take them from the chest then drop them again they may have exactly the same "shrinking away into nothingness" time as white items.