I think this is the 'It's either 5-stars or 1-star' principle in action. I feel given a different context they might say that both both renditions are good but one speaks to them on a more personal level.

I'm guessing this comes about as a desire to be heard causing a magnification of the opinion rather than the force of the opinion. If you feel like nobody takes notice of your moderate opinion one might feel like a expressing stronger opinion in the same direction might increase the impact.

I also just don't care for Buckley's cover, and it's not a "not quite as good as the other one" feeling but a "not good" impression, so I can see where GGP is coming from. Buckley needlessly draws out the song.

I own both Cohen's track from Various Positions and Buckley's cover from Grace, which Google gave away for free to try to convince people to use its music store. I don't listen to the latter. Even if I didn't have the former, I still wouldn't listen to the latter. That's just how I feel about it, and I understand that other people feel differently.