Dragon Knights... It's not the story itself, but the artstyle. I hate to be rude to the illustrator, but the artwork is terrible. Don't believe me? Go to Bing or Google Images and type in 'Dragon Knights Mineko Ohkami' to see what I mean.
I stumbled across a yaoi called Okane Ga Nai. I'm fine with yaoi, it's okay, but... but... it was... just... no. It gave yaoi a bad reputation in my book. Nothing can redeem Ayase. He single-handedly ruined the entire show. An extremely submissive, weepy blonde big-eyed pretty boy who can't raise a finger to defend himself and is probably the personification of the word 'uke'. The proportions also make me cringe. He looks like a kid compared to the 'seme'.
I don't like anime/manga that have an "unbalanced" element.
-Full Moon is okay, the main character, Mitsuki, acts a bit like a Mary-Sue in the beginning.
-Orange Planet, too repetitive of a plot where I lost interest in it.
-Somewhat of Fruits Basket (*Gasp!* How can you say that?!), It is Tohru Honda... I think she assumes the personality of Mary-Sue a bit. I expect flames but that is my opinion.
-Romeo X Juliet (Yes, the manga version), it was a slight dive into la-la-la land toward the ending where I felt some of the originality was lost. It has been awhile but that tree of life spirit thing seemed to be a last-minute put-together to make-up for the Montague and Capulet lovebirds to have the death scene. It gave me an unsettling feeling.
Those anime or manga that give a character too much "slack" to be a realistic expectation for their actions (ex: When the girl hits the guy for an honest answer and she is automatically in the "victim" category. Then the story revolves where the reader is supposed to sympathize with that. Nuh-uh. Too desperate. There has to be a give-and-take and it feels one-dimensional).
When an anime/manga does not have substance. I can not recall any at the moment.