As far as 'looking into the backpack', I hope its something operatorable entirely from the keyboard. For example, you could have a universal 'action' key that does things like activate switches, doors, and special items.
I could forsee an numbered list appearing on the screen when hitting such an action key while standing over a discarded pack. Perhaps the list is organized by weapon type with number of extra clips being listed beside. An example might be:
1. m9 (3 clips)
2. mp5 (1 clip)
3. HK69 (0 shells)
Since everyone starts with an m9, I would only take clips by hitting '1' on the keyboard. Each time I hit '1', i would take one clip. Lets say that I didn't already have an mp5, then hitting 2 would first take the weapon (along with whatever is already loaded into it), and successive hits would take one clip each.
So, emptying the entire pack would take a total of six keystrokes: '1' three times, '2' two times, and '3' one time.
Once i'm finished, I could either hit the action key again to go back to my previous weapon, or hit the movement or fire keys (should I be surprised).
A system like this ensures that you have the abililty to choose what items you want and what you'll leave behind while still allowing you to quickly bring up your weapon and retaliate against any incoming threats.
I guess what I really don't want to see is something that involves the mouse since that means going from inverted mouse control to normal as well as dealing with a mouse speed that might be completely different than what your look speed is.