And I still don't agree with the comparison thing. Being an "FPS with RPG elements" is a hugely tenuous link. Borderlands is closer to Diablo in terms of similarity and FO3 closer to System Shock. Both of these comparisons are tenuous but both also have as much in common in terms of core mechanics.
I don't see it as 'hugely tenuous':
-Both games are played from a first person perspective
-Both games feature you running around shooting things
-Both games feature a (relatively) open game world
-Both games feature multiple concurrent quests
-Both games feature an XP system and levelling up
-Both games feature skill points you can invest in skills
-Both games feature loot you collect
-Both games feature currency, buying/selling gear etc
-Both games feature NPCs
-Both games feature a wasteland environment quite extensively
-Both games feature fast travel
Also for the purposes of making a comparison, both games came out within the past couple of years, whereas Diablo and System Shock weren't even released this millennium, so it's an easier point of reference for 'newbie' PC gamers. If I had to choose some games to compare Borderlands to, FO3 would be one of them, and clearly there must be something in it since I'm not the only person to make the same comparison.
No, they are not two peas from the same pod, it's not like comparing say FIFA with PES. But equally when it comes to 'hybrid' games like this, it's not like there is a huge number of more fitting titles for comparison.