It really just was one of those years, wasn't it? The ones which see a particularly high volume of seriously great records out - 1977, 1989 and 1994 are three others which spring immediately to mind.
I mean, it wasn't just the quality of the albums but how it spanned various genres too. I'm recalling this from memory so I might be wrong, but for forward-thinking electronic music you had the likes of This Bliss, Untrue, From Here We Go Sublime, † etc; over the pond there was a slew of colourful, dancable alternative stuff, real career bests from Of Montreal and LCD Soundsystem; esoteric underground crossovers like Mirrored, Person Pitch, Strawberry Jam, All Hour Cymbals; grandiose alt rock saw Neon Bible, Let's Stay Friends, The Boxer; even with mainstream chart indie you got some standouts like Favourite Worst Nightmare and Because Of The Times. And that's not mentioning M.I.A., Black Lips, Deerhunter, Stars Of The Lid and prolly loads more I can't recall.
I felt last year was a great and similarly varied one too. Not quite up there with the ones listed but could be regarded as a year of increasing vintage with time.