Yotsuba is indeed awesome. Vol.4 isn't until august?
Well, here's some stuff I read which you might enjoy:
--20th Century Boys. Good story, good characters, fast page turner. A little depressing and if you're looking for humor stay away. Kanji level = medium, vocab level = medium
Monster is another page turner, but I'd go with 20ch boys over it.
--Fruits Basket. Really good writing (the manga-ka has one some awards for it) ok story, great characters, slow and hard to read at times and closer to reading a novel than a manga, but it's really a nice read and great for learning vocab. Also it's pretty down to earth especially int terms of it's vocab. There is a good amount of humor, but there's also a lot of seriousness. Kanji level = low, vocab level = high
--Trigun Maximum. Umm, not sure if I'd recommend this anymore. Really good art and great action but the story is nearing the end and it's turning into a freaking videogame plotwise. Also it's a tough read becaues there are tons of high level and uncommon kanji. The characters are great, but yea...I don't recommend it too strongly. Kanji level = high, vocab level = medium
--Berserk. Excellent fantasy series. Great story, great characters, jaw-dropping art. At first it seems tough to read in terms of Kanji level, but when you get used to it's vocab it reads pretty quickly. One of the best manga I've read, recommend it strongly. Kanji level = medium/high, vocab level = medium
--XXXholic. Great CLAMP writing, great CLAMP art, great CLAMP characters. Basicaly a collection of short supernatural tales aka. The Monkey's Claw. CLAMP stuff is always great to read because it uses very simple vocab and kanji but yet it makes very poetic dialogue out of it. Kanji level = low, vocab level = low
--Suki Dakara Suki. More CLAMP. Short 3 volume series that's quite likeable and has great art and characters. Also very down to earth-ish. Like above it's easy to read and fun.
--X. Again more CLAMP. But umm it's kinda on hold and may never be finished so at this point I dunno if I'd recommend it.
--Gouhou Drug. More CLAMP (yea I'm a pretty big CLAMP fan
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). If you don't mind a little bit of homosexual undertones, this has great art and solid writing. Actually I recommend anything CLAMP besides Tsubasa I guess
I know you said your not into the Jump stuff, but I'd recommend giving various Jump mangas a try as they are excellent for that "down-to-earth" vocab/kanji practice and for every 5 cliche shounen action mangas there is one great manga when it comes to Jump stuff.