Like many others already said, it would have to be complex, interesting, and rather fast paced (at least, not dragging) plot with twists, good character developements, and depending on the type of manga, good jokes scenes that would make me really laugh out loud. I think that coveres it all for me.

What does nova0803 think of the responses? Has it helped any?

Alot was already said ...
My two cents is to pick a genre first. Then write to target the audience that would read it (depending on your genre, lesbians may not be applicable!). Excellent artwork is a must since it is an artistic medium, and also to draw people in with cover art and whatnot. Also, I feel like if you aren't Japanese and you are trying to create a manga, you really have to win some people over with good artwork, because some may not consider you because its not "authentic made-in-japan" manga. (Does anyone else get this sensation?)
And of course since it is also a written story, the story has to be good. In particular characters should be well hashed out with positive and negative points like a real person would be; settings, times, etc explained well, especially if its sci fi or fantasy; and it needs flooooww. That one really turns me off from a manga, where the story idea might be interesting and the artwork good, but the characters and scenes are just all over the place - its confusing!