Borderlands 2 uses the usual achievements, but it has its own in-game achievement system in the form of challenges. Challenges award you 1-100 points of “badass rank.” Increasing your badass rank awards you badass tokens. Badass tokens can be redeemed for a diminishing bonus to a stat such as accuracy or gun damage. Badass rank and its bonuses are account-based, but challenges are character-based. If you play the game again, you may notice it to be marginally easier with +5% to everything, but then you thousands of badass rank to approach “+5% to everything.”
Pretty much everything you do contributes to several challenges, with the note that “everything you do” almost invariably involves shooting people in the face. One shot could potentially add a tick to getting criticals with submachine guns, killing enemies with submachine guns, killing enemies with criticals from submachine guns, getting a second wind with submachine guns, getting a second wind from killing badass enemies, killing midgets, dealing corrosive damage, and maybe a few more. That lucky bullet, of course, would need to be from a corrosive submachine gun, killing a badass midget while downed. There are also exploration, loot, and boss challenges, and the inevitable meta-challenge.
The math becomes interesting at times. Few enemies survive multiple headshots from a good sniper rifle, so you will probably complete kills with sniper rifle criticals before you complete sniper rifle criticals (which demands higher numbers), unlike other weapons. As Zer0, kills while using his special ability come slowly because you get one attack per special ability use, while the Mechromancer can hit F and go AFK while psychos die. Level does not matter, so you could round out your challenges by going back to a lower-level area and picking off enemies that cannot hurt you. Of course, that is putting in a lot of effort to get a few points that will eventually contribute to 0.5% bonus, but you are MMO players who grind fiercely for best-in-slot. Also, I accidentally earned quite a few points by putting on a spike shield (thorns aura) and walking through a spiderant nest (tiny enemies that pounce on you). It is enormously satisfying to watch the tiny, annoying enemies immolate themselves for having the audacity to threaten you.
Also, when you complete a challenge as the Mechromancer, she sometimes exclaims, “The completionist in me is like, ‘YEEAAAH!'”