Randomization

Randomization, along with Customization, is a HUGE part of borderlands.

Map
Almost all of Pandora will be different each time you play through the game. Depending on what difficulty you choose, there will be less cover for you and more cover for your enemies, or the stairs to the next area might be better gaurded and further away.

Main Roads and Cities
Pandora cannot always be changing, you wouldn't know where to go! Gearbox has planned that all main cities and roads will stay in the same spot in each playthrough, to ensure you don't get too frustrated.

Enemies
The enemies in borderlands are always different. Sure, bandits might mostly use SMGs and shotguns (and scrap-metal armor), while the Crimson lance uses assault rifles and shields (with mint-condition Crimson Lance armor), but each and every enemy has a few things different about them.

Creatures
Creatures have many different powers, some will simply headbutt you while others shoot fire.

Equipment
Every piece of equipment is different in Borderlands, even by the tiniest bit.

Weapons
Every single weapon an enemy carries is different. Some shoot explosive round, some are more accurate. But every one is different.

Armor
You'll never see a bandit in the same clothes as another (however, since the crimson lance wears a uniform they may look the same).

Weapons
Every single weapon an enemy carries is different. Some shoot explosive round, some are more accurate. But every one is different.

Treasure Chests
Treasure chests are usually found after a battle, and contain weapons. Every chest is different, they may only have one rocket launcher, or they might have fifteen different pistols.