The first true disappointment for me was when I saw the Sunrise/Sunset covers for the very first time on an Asian music site. I wasn't following Ayu much at the time, still new to the fandom and so far, most of what I'd seen of her had been impressive so seeing those atrocities was like a swift kick in the gut. Then the song and PV didn't turn out much better so I just look at it as her worst single overall for me as I like nothing about it.
The biggest disappointment for me however is hands down the RnRC album. It was the first Ayu album I got to anticipate and preorder before it hit stores. I loved Microphone, Don't Look Back, and Sexy Little Things and the covers and promos were wicked awesome. Then I heard the final product.
I think now, I occasionally listen to Microphone but the release overall was a complete waste of time and money. I had the choice of getting that or a Pink Lady Greatest Hits CD at the time and still wished I had went with the latter.
(n I dun curr whatchu bishes say LOVE'N HATE is flawless, k? kthnx *hurflip*)
EDIT: Also, another big disappointment was after watching three concerts, I already knew Ayu's concert pattern. Oriental section-->rockish storyline section-->ballad with love story section--->cute, kawaii desu~ section---> party, let's all chant and be genki! section. *facepalm* Not that I don't like any of these things but it's just SO much of the same. Her setlists suffers from that sameness too, every time I see evolution~SURREAL~evolution I cringe. There's no reason someone with as big of a discography as Ayu should have certain songs or mixes popping up on just about every setlist. She has CDL, a-nation and at least one tour a year, it won't hurt to leave B&G, evolution, and SURREAL off of at least ONE of them.