There are mangas that are just too darn LONG. Wherein they just seem to drag on and on because of scenes/updates that doesn't really impact the story. They're OK once in a while, but they can get real old real quickly so mangakas should use them sparingly.
Then there's the issue of RUSHED mangas. This character is super evil and hell-bent on getting his revenge and then suddenly he's all "I'm-doing-this-'cuz-I'm-sad-etc." I'M BE LIKE, "UGH....WHY????" I think the main reason for rushed mangas is that the mangaka's got an idea, he decides to draw a manga...everyone's happy. That is until he runs out of ideas so he just repeat scenes again and again and AGAIN and AGAIN and AGAIN and AGAIN and AGAIN....
OK IMMA' GO NOW 'CUZ MY BRO IS ITCHIN' TO USE MA' LAPTOP 'CUZ HE DIDN'T BRING HIS AT MANILA.