Well, seeing as how you deleted the link while I was typing, I'll paste it down here for you so that you can get some more insight.

A few factors, but not an exclusive listing:

• Characters: If the people involved aren't excellent, then the anime has little chance of making a 10/10. And note that I said "people". The writing has to be good enough that I stop thinking of them as characters on a screen and start thinking of them as actual human beings.
• Music: Without exception, every single one of my 10/10 titles has an excellent soundtrack. For some, this designation is obvious. For others, much less so. Fortunately for me, I have just as eclectic a taste in music as I do in anime.
• Finish: It has to come to a believable full stop at the final episode. It may be the easiest thing to happen, but it's still a mandatory point.
• An ideal of the genre: When I point to something and say that it is the best possible example of insert-topic-here, then it is a candidate for a 10/10. Regardless of what that topic might be, all of my 10/10 titles hold at least one of my best-of choices. Those examples may repeat within the various 10/10 titles, but I never demote a title once I give it a top score.
• The extra mile: It's the hardest thing to quantify, but the one thing that will get a title up into this range is to give me more than I was hoping for. The scales have to be grander. Or the romances have to be dearer. Or the action has to be harder. Or the topics have to be deeper... It's not enough to just show up with a good title and have them expect a 10/10 out of me. They have to take the next step and give me an anime that is well beyond what I had any right to expect.

It gets more involved than that, but that's the basic nutshell version.

For examples of 9/10 that failed to get the promotion, I can just look at last season's offerings.

• Akagami no Shirayuki-hime: Honestly, if this had been a straight-up two-cour show, it might have made it. But splitting the story like that does tend to make me hesitant when giving out the top marks.
• Gatchaman Crowds Insight: While it passed a lot of the requirements, it still didn't make it beyond the character question.
• Non Non Biyori Repeat: Excellent as it was, it just couldn't go the extra step into absolute greatness.
• Ore Monogatari!!: The ending was what killed its top mark. If they had made the challenger appear an episode earlier so that they didn't end it on that note, it would have been a much better show.
• Wakaba*Girl: Face it. As much as I did love it, it was a fairly simple show done exceptionally well. And those don't get full marks.
• Working!!!: Again, no full stop. It leads directly into a final OVA. And without that sense that the anime has ended, there's no upgrade.