The grenades were much, much better. The Soldier/Commando turret was a lot better. Melee combat with Zero was awesome compared to Brick in the first game - "Many Must Fall" is one of my favorite character skills in any video game, ever.
They really messed up nerfing the chests and vendors so drastically and focusing on the raid bosses that dropped shitty loot as well. I read somewhere that the typical raid boss had a 1 in 10,000 chance of dropping a Legendary but even then, it might be one or two levels below your level.
The payoff was so infrequent that I just started making my own gear with the Gibbed program and I never even considered modding in the first game.
I don't think they ever really had a good handle on their own exponential leveling system and they never planned ahead for how the game would play beyond level 50. Gearbox is certain cranking up the difficulty was the right move but I think they ran a lot of people off with that.
BTW, how is Roma Tomato Nose Cat? Sparkles does not want to go outside these days because the weather has been so cold!