As far as what the detractors say, I can't really disagree on any particular points they made about why the game isn't as good as it could be. (Except for Cutter, who just hated it, and the other guy who hated Cutter for hating it...)
That said, I had an absolute blast with Borderlands, and still do when I fire up a 4-player LAN session at home. It's sad to think how great this game may have been had Gearbox given it the proper depth and ported it properly to the PC. Eagerly awaiting Borderlands 2, all the while fully expecting it to have glaring faults similar to the first.
“The greatness of a nation and its moral progress can be judged by the way its animals are treated.” - Mahatma Gandhi