It's just nuts how much detail every single gun has.
Yes I've got a Maliwan SMG at the moment that can switch between incendiary and corrosive elements - (alternate fire modes being one of many great additions to the game, one of the pains in BL2 was trying to make sure you had at least one viable weapon for each element in your arsenal at any given time) - the animation and effects when switching between the two elements are sublime.
It could just change an icon on the screen from orange to green or something, or the ammo count could change colour, but instead there's a gorgeous little animation as your character flips the switch on on the gun itself and it ripples from orange to green (or the other way around), and then does a sort of flash along the barrel and on the control switch, with a solid block of colour at the back of the gun which has the ammo count set into it, before settling down to its new element. When zoomed with it the crosshair circle changes colour and there's a superimposed HUD element icon that you'd see if staring down the barrel.
A small detail on a single gun, but the amount of care that's gone into them all is really cool to see.