What breaks the monotony is that you are constantly leveling up and looking for the best weapons and equipment. You seem to have been using the same weapon or type of weapon the whole game. There are so many different guns with different characteristics which you should have been exploring. You can also use your character's unique action skills too, and experiment with the skill tree to try different play styles.
Yeah, I was changing weapons fairly frequently, but they were pretty much just more powerful versions of the gear I was already using (ex. a sniper rifle or submachine gun with an attack of 100 and lightning damage rather than one with 89 damage). There was really no reason to use anything else, and changing guns didn't make the experience
different from what I experienced. The scenarios really didn't change much, and the enemies pretty much attacked with the same patterns the entire game.
As for my character's unique action skill, I played a Siren and at the time I was playing Gearbox shipped the game
broken. The Siren's unique ability to deal mega damage while Phase Shifted didn't work. Instead, you merely dealt your normal melee damage while shifted, so the ability was useless. And by the time I discovered that, there was no way I was starting over with a new character and going through that tedious experience all over again. By the time they patched the ability to actually work like it was supposed to, I was long since done playing.