Never played Borderlands but I strongly disagree with several parts of your post. Sticking a Diablo 2 loot styling into an FPS/RPG was revolutionary. Its the default design of the industry's biggest games now but no-one had done it before and no-one, that I can remember, was doing it back when Gearbox decided to. Diablo had existed, successfully, for over a decade at that point and other genres were not copying it, and they still aren't- they are copying Borderlands.
I can't comment on the gameplay or humor, although I remember the usual places cringing about Reddit references. More to the point, while not necessarily our style, thethey are exactly what the general audience wants. The gameplay loop is almost scientifically designed to keep people playing longer than they would an average game and as to the jokes, well... there's no accounting for taste. Hell, lets be honest, your average game has pretty awful writing, I full believe that no matter how cringey the occasional meme reference could be, its better than the industry standard. And yeah, the art style was plagiarized, its one of Randy's calling cards. But remember, he never as far as anyone was aware, had to pay out to the artist and the art style, in the public's mind was a big hit and became iconic to the series. It was a big win for Pitchford, a big scummy win that probably would not have come out of another studio head.
No offense, but I think you'd have to play the game to realize it wasn't revolutionary. Conceptually it was fine (not revolutionary, but fine) but in execution it was boring. The weapons are randomized but not really in a fun way as they all boil down to <weapon type> with a chance to do <elemental damage> with random modifiers added on top (extra bad recoil, bonus melee damage, random number for magazine capacity, etc) but nothing that effected the gameplay like Diablo 2. In borderlands games your are
nearly forced to carry one of each weapon type (fire, shock, corrosive, in BL2 slag) in your four weapon slots, a problem that gets substantially worse on higher difficulties making the game less fun and each character called for specific weapons inside their skill trees (Maya has SMG skills, Zer0 has sniper rifle skills) so there wasn't a ton of deviation there. If you were level 30 and had a corrosive SMG you would be looking for a level 35 corrosive SMG at level 35. Compared to Diablo, which had a number of items that dramatically changed gameplay for all classes.
Speaking of classes, the classes in Borderlands are also garbage. Each character had
one active skill and it was usually a very passive one at that, the rest of the skill trees (of which there are 3 per character) were full of passive skills, timed skills (when you kill an enemy do 4% extra damage for 2 seconds), and skills that added on your passive with more passive effects (example -
two characters in the game have an active skill of "deploy" a turret. One of the "ultimate" skills is "the turret also heals nearby allies and gives ammo"). Diablo 2 (a game nearly a decade older) had much more fleshed out classes and the different trees fleshed out the people within the same class well enough.
The humor is really god awful, I know you haven't played it but if you've played World of Warcraft, think of the worst writing there and you'll have borderlands' writing. I specify WoW because it has the same problem, a small joke dragged out (WoW has an entire Indiana Jones themed area completed with several appearances by "Harrison Jones", after he appeared several times in previous expansions).
TL;DR - Diablo 2 perfectly mixed with a FPS would be revolutionary, but Borderlands isn't that game. Borderlands is a basic, mediocre co-op game that by
chance released when fatigue was starting for the non co-op modes of other shooters (Deathmatch and Team Deathmatch). It lacks real depth compared to D2 and really any other game as an RPG and lacks real depth when compared to any other shooter (co-op or otherwise) from it's time or earlier. It largely gets a pass because one time through
and with friends it's "OK", but even then only on the first difficulty level.
I would posit that
any game can be made good with friends.
EDIT - Also, any "very cool" gun is one you were likely to never use or see, as the "legend tier" drop rate was unbelievably low (1/10000 per lootable instance, but inside of a very deep loot pool that also includes relics, shields, class mods) and even if you got one, it would be tied to your level and would have to be one that your character was able to spec into (found an orange shotgun on Mordecai? tough luck.)