Rama, you sick sadistic bastard...how awesome looking is that mockup! IT'S JUST AS I REMEMBER IT!
Anyways, Rama nailed it in that it was the semi-opposite approach to quality control that OCR has. Instead of keeping quality high with evaluations, submissions, judges, and all that jazz, people could basically upload a song of theirs every two weeks or so (that's what I remember it being) and it would automatically get posted on the site. Of course, I don't believe you're allowed to upload songs that contained any other copyrighted works (example: I don't think the Snoop Dog/Kirby mashup would fly) or midirips+drums and the like. The songs would then get reviewed and given points, and end up in tiers ranging from Tier 1 - Stuff of Legends, all the way down to Tier 7. This itself had a couple of problems based on design, such as a song would sometimes not have any reviews and it would sit in Tier 4 forever, and that there were a LOT of people submitting who were just getting their musical foothold. I know there are at least a few OCR people who came over from VGMix after the site went down, and quite a few got started there since the submissions here had a different approach. There were WIP sections as well, but you knew that once you were personally done you could upload the song and not worry about it being posted up unless you infringed on those guidelines. You just had to be personally satisfied and also be able to weather the reviews.
It definitely provided a balance and alternative to OCR that was extremely healthy, but since it came down there really hasn't been anything to really take its place in that niche role...
Whatever happens, it's good to see VGMix interest is still there outside of DoD!
Edit: Should mention the reviews were done by any member of the VGMix community, not a select group within the members. Anyone with an account could review a song. The reviews themselves could also be given a thumbs-up or thumbs-down, if memory serves correctly...