Art. Hands down. A sort of glimpsed overview of the first page or so of artwork. Not so much hair or anything specific but how the characters in general look.

That's just what I immediately notice, anyway.

Hi Ryuu :P

The main genera, shouju, romance, comedy, etc. That is what usually decides it for me, that and the artwork quality.

Hmm, since this seems to be mentioned a lot, I'd like to add that if the dialogue/plot doesn't have me snagged within the first chapter (maximum first volume, if it's a popular series/something I should supposedly be interested in), then I am not going to be snagged. But the question was the "first" thing I noticed… which has to be art.

Even before that… the cover. I know, I know, don't judge a book by it's cover blah blah but especially for certain genres yaoi, an eye-catching cover is a must.

i notice that a lot of people say the storyline, and this might just be because i read books as much as i read manga (well a lil less now), but i find i can usually tell whether the story is going to be good from the first 1-5 bubbles, usually simply the first verbose narration box. i guess i never noticed that this is probably quite odd. (i mean i've never got into one piece, and i wasn't that estatic about the first few bubbles (that and there are lots of hats, i don't feel comforatable when i can't see character's hair....))

Actually mostly don't notice art first in a manga, you notice first the title, then the art, then the genre, then the summary, and after that you'll see how many chapters is there, whether it's done, delayed, cancelled, or on going, then you'll read it and you'll notice how it was made.

I notice the main character and the plot setup from the beginning of the first chapter (first few pages).