It's like Rainbow Six Vegas... with 17 million guns.
It's like Diablo 2, Torchlight, Dungeon Seige 1 and 2, or Titans Quest (take your pick) with GUNSSSS. And in first person. Borderlands is a First Person LOOTER.
That's it. You run around shooting things, not because you want to advance the story, or because it's a compelling cinematic experience (COD4/5/6), but because you're looking for BETTER GUNSSSS that go PEW PEW PEW better than your old guns.
If you don't like loot hunting games set in a sandbox world with no one to hold your hand and tell you where to go, then don't play this game. Go play a linear corridor shooter like Modern Warfare 1/2, Gears of War 1/2 instead.
If you get bored easily without the game constantly showing you where to go next and who to shoot, don't play Borderlands.
Borderlands = Sandbox game with lots and lots of loot (guns mainly). The story is not the main point of the game, it's the item finding.
First Person Looter. Got it? The "RPG" part of Borderlands is that they added loot to find. And GOD DAMN if it isn't exactly what I wanted to play for years. This game F'n rocks.
You can have your linear corridor shooters. I'll take sandbox games, thank you.