One problem with a manga is that it's often written in Japanese. This means that I won't even be able to begin to translate it, because they use glyphs in their writing system. If it was written in the Latin or Cyrillic alphabet or even the Japanese kana scripts, I wouldn't have as much trouble. But I don't know how to read symbols that stand for a whole word or part of a word, so I wouldn't be able to use a dictionary to translate it.
Another problem is that it's in black and white. Sometimes that's okay, but other times it's a nuisance. If it's too detailed, but in black and white, sometimes it's very hard to tell what you're actually looking at. All you see is a cluster of lines. It's a bit like those vision tests where you have to look at a page of dots and try and 'find the hidden triangle' or something like that. Except this time, you're looking for the hidden characters.
I have seen a few erotic manga, which I enjoyed of course, but I'd still choose comics any day. I like all the colors, the soft rounded lines, the fact that I can actually read it... They just look so much more friendly to me.