I have varied tastes, though I tend to lean towards shojo manga.
I often buy manga for the romance. I am romance obsessed. But I don't like angsty romance manga, or angst in general.
Humor is a great plus, though not entirely necessary.
I like manga that have both incredibly likable and incredibly hateable characters. Such as Fruits Basket. Where I like characters such as Tohru, Momiji, Saki and Kyo, yet hate characters like Shigure (From time to time. Conniving jerk.).
Great villains are a must. Whether the villain is someone I love to hate, or just love. My favorite villain is Akito Sohma. So twisted.
Original storylines are great, that said, I have no issues with cliched plots. Even if the basic plot of a story is cliche, the storyline itself might be original. The most overused plots can be turned into the best stories.
I love characters that have plenty of depth. One-layered characters are just boring.
Of course, the best characters are the weird ones, the kinds that you don't usually come across in your typical manga. Such as Naka Kaburagi, from Nosatsu Junkie. (Though Umi is great as well.)
Also, if a manga can make me laugh or cry, then I know it's good. I love manga that I can't put down until I finish.
Well... That's about it.