This article is fairly short, well sourced and largely a good summary of the subject. I have some comments below which mostly relate to the clarity of the prose and some minor POV problems (which are sometimes unavoidable when the sources all describe the event breathlessly). They should be pretty easy to clear up and the article passed in short order. Sorry for the long wait.
If we're going to include all the themes and event chairs in the body of the article we should make mention of them in the lede.
" It marks the grand opening of the Costume Institute's annual fashion exhibit. It is celebrated by an evening of fashion to match the theme of the exhibit." The sentence structure of these two should be mixed up a bit (to avoid starting both with "It *verb*"). Alternately the last sentence can be folded into the penultimate one. Other passages in the article have the same problem.
"It is one of the most notable sources of funding for the Institute" I recommend splitting off the fundraising bit in the previous sentence and integrating it with this one (assuming that the bulk of the fundraising at the ball goes to the Institute)
"In 2014, the individual tickets cost $25,000 for those outside the official guest list..." The nymag source notes ticket prices ranged between 5 and 10 thousand. I think we're trying to stick to the facts but there seem to be three points of interest: the ticket prices in 2014 after the hike, the ticket prices immediately before the hike and the ticket prices in the recent past. Might as well spell out all three.
"Following the event, the exhibition runs for several months." Is this better placed at the top of the section? I'm not sure, but it seems a bit odd to detour thtough the exclusivity only to return to the exhibit at the end.
"During the cocktail hour, guests arrive to walk on the red carpet tour the years special themed exhibition and are seated..." There's a word missing here, or some punctuation at least.
It's just a bit much. The article is constantly straining to not wax rhapsodic about the event. The issue is mainly the tone of statements written in the encyclopedia's voice, not the literal summary from a reliable source. Protonk (talk) 23:08, 13 October 2014 (UTC)[reply]
We could just say that vogue calls them the preeminent entertainers of the day. Or we could list a few from certain years. This is relatively minor and it's the last remaining concern which I feel still relates to the criteria (POV), so I'm ok with passing the article and leaving this as a suggestion for you to change or not change at your discretion. Protonk (talk) 14:15, 14 October 2014 (UTC)[reply]
I won't fight the Themes section although I don't see the value of lists like this. I do, however, want to be convinced that the honorary event day chairs are important enough to enumerate.
Thanks for reminding me that wikipedia is a tertiary source. I'm aware the statements are sourced. I'm less convinced that we need to have an exhaustive list of them. Why are they important? What can we say about them aside from enumerating them? Protonk (talk) 23:08, 13 October 2014 (UTC)[reply]
But a list of who is a co-chair of a charity ball is not a summary. I passed the article, as I think this is more a matter of opinion than a GA requirement, but looking at the talk page I see Star Mississippi has the same concerns about the list as I do. It's worth considering that we may not need it. Protonk (talk) 14:15, 14 October 2014 (UTC)[reply]