[Borderlands] is only good once. I play Space Empires 4 more over a period of time because it's random, I even gave Diablo 2 more time than Borderlands. These games have random elements missing from Borderlands.
From the sound of it, you weren't a huge fan of Diablo 2. If that is the case, it is to be expected that you found Borderlands to be a major disappointment.
I've played countless hours of Diablo 2, so it is the standard by which I measure Borderlands. There are a few key areas where Borderlands disappointed me.
1. Sparse skill trees and a single special ability. Some of Diablo 2's greatest strengths were its skill trees and abilities. They were easy to understand, yet offered a great variety of choices. Each character had at least 2 or 3 tried and true cookie cutter builds and any number of customized variants. With Borderlands, each character has essentially one build with slight variations. Lack of variation and customization in skill trees is a very real reason to only play the game a few times.
2. Lack of customizable items. In addition to rare, unique, and magic items, Diablo's socketed, runeword, and crafted items added an extra level of customization. The necessity to collect not only weapons, but also armor, rings, amulets, and charms increased the desire to spend endless hours hunting for loot. The sheer number of guns in Borderlands is nice, but there is no way to create custom items. One little character mod hardly makes up for all of the various items Diablo 2 offered.
3. Low level cap. Diablo 2's level cap of 99 was so high that it couldn't be reached without excessive playing above and beyond a single play through all 3 tiers of the story mode. It gave players a reason to continue to improve their characters after completing the story. The level cap in Borderlands is too low to encourage high level characters to continue playing.
At the end of the day, at its core Borderlands feels like a new version of Diablo 2 that has been dumbed down for the console market. All that said, the game can still be a lot of fun to play with friends
. If you don't intend to play it cooperatively with friends, you won't get much out of it.