I second: Gin no Saji, Emma and Gokusen

You could also try:
Can't See Can't Hear But Love
Classmate, Kamimura Yuuka wa Kou Itta. - so far it was really good (it may have a hint of ecchi though)
Immortal Rain
Maybe Karin

I would also recommend some other series, but...
Otoyomegatari - it's great, but some would consider it shotacon, so I guess that's a no-go
Kiba no Tabishounin - The Arms Peddler - really good, but I guess gore counts as mature
Please Save My Earth - some would say it's shotacon and it definitelly had some mature parts, though the story was rather good
Hoshi no Samidare - it had a little bit of ecchi in it and some would say it's lolicon, but it was so awesome
Fujimura-kun Mates - a really funny gag manga, but it runs on ecchi jokes (though, all those jokes are pretty much verbal ones)

Bump!

After reading your first post, spontaneously I had to think of Gangsta., then a few seconds later there were the name SHIMOTSUKI Kayoko and - last but not least - Yume Miru Taiyou, because I enjoyed reading this one a few days ago casually and you mentioned "no mature storylines". So, maybe this could be helpful? ^^"

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"It's like love is a bonus in your life. Even without it, I'm just fine. I don't feel like pointlessly searching for it. You don't have to make yourself unhappy." (Ookami Shoujo to Kuro Ouji)

"Why does it rain?" - "To create drama." (Drops (Andou Yuki))

Mars
P.A.
Team Medical Dragon the first chapter is not representative of the whole thing
Mononoke + anime
Hana no Namae I suppose you could call this deep...and dark!
Warau Kanoko-sama
V.B.Rose
Bitter Virgin
March Rabbit
Nineteen, Twenty one
Please! Mr. Bear! I think this is what you're looking for?
Haruka 17...I guess this may count as mature, but it's a good story
Liar x Liar
Shibatora
Ao Haru Ride and Strobe Edge are both pretty unusual for shoujos
Yoakemae
Peak (Im Gang-hyeok)
Dr. Frost
Katana
Gakuen Babysitters this one is like Love so Life

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I am the master of my fate;
I am the captin of my soul.
William Ernest Henely

I rate manga with 5 being the average score.