Loot. It is the desire of every fan of role-playing games. The need for better stats, the want for something that looks cooler, the desire to know that your items are better than your friend's.
Guns. The addiction of the shooter player. They come in many shapes and sizes, but they all serve one goal: killing.
The original Borderlands gained a lot of praise for combining the shooter with loot-based RPGs like Diablo. Developer Gearbox Software basically invented a genre. And now, it nearly perfects it.