"There's nothing like having your friends show up with lots of guns"
Stuck on a boss? Have a friend (or 3) jump in the game with you and unleash a never ending supply of firepower. Still stumped? Choose a different weapon, you have a few million to choose from. Everything from pistols that release a burst of electricity to rocket launchers loaded with corrosive rounds. All that wrapped with character customization to allow you to craft a unique experience, adding to replay value.
For example, my friend and I both play as the soldier. We have completely different skill sets that allow me to focus more on healing and support while he has the more combat driven skills. Since there are 4 characters to choose from, it's not hard to see how you are able to forge your own path allowing for you to maximize your play style.
Everything about Borderlands appeals to me. Memorable characters, bandits yelling because I just lit them on fire with a machine gun, wise cracking robots, the list goes on and on. Anyone that hasn't at least taken a small jaunt in the Borderlands should be booed and shunned by his/her peers and taunted until my throat is raw.