Loot and shooting.
This is probably the only game that I've played so far that I've been thoroughly okay with a lackluster story. In fact, the main story is so flimsy, that it might as well not exist. The real star of the show here is the loot. All 87 bazillion guns. Most are simply variations upon each other, because there are only 8 or 9 types of guns, and 3 elements (not including neutral). They'll vary in firepower, elements, accuracy, and firing and reload rates, and you'll often find yourself caring more about the next awesome gun you might get instead of the quests.
Let's talk mechanics. Truthfully, the RPG mechanics are really simplified. You'll only equip 1 armor (the shield), but you'll have a ton of class and grenade mods to play around with. On top of that, there is a skill tree, although it truly only has one active skill. Aside from the action skill, the remaining skills are more passive, affecting cooldowns, buffs, etc. This, however, keeps sifting through skills to a minimum, and allows players to concentrate on the gunplay. It would be a very good game if it didn't have solid shooting, and Borderlands doesn't disappoint. There are a variety of enemies, most of whom attack in waves, forcing you to keep yourself on your toes. All enemies will have a weak point which will reward you with a critical hit if you manage to shoot it. What's different is that a headshot on a human may not result in an instant kill (or it may if you have a strong enough gun). But since Borderlands relies on number crunching in the background, this makes sense. Overall, though, it doesn't detract from the experience.
What does, though, is some of the oddly foregone potential in the game. I know I said that the story doesn't really matter in this game, and for the most part, it's true. It comes pretty sharply at odds with some of the funniest characters I've seen in a long time though. They've really built some pretty interesting personalities into the game, but hardly any use for them. In fact, the quest descriptions get lazy after a while, and simply rely on the title for the description. The great characters also come starkly in contrast with the other NPCs in the town(s). Most of them are like traditional RPGs. They stand in one place and say the same exact single line no matter when you talk to them. While it's not a huge problem, it does break the flow of the game a bit.
There's also a ton of replay value in this package. There are 4 characters to try, as well as a 4 player online coop. I've personally played as the Siren, and I haven't had the chance to play online yet, but I'm hooked. I'm completely psyched for playthrough 2, as well as the expansion packs that were released. If you love hunting for better equipment and love shooters, you should definitely take this for a spin.