Nana is the most popular girls' manga now being written. Its author was also voted favorite overall mangaka in the last Oricon ranking.
It's also had two successful films and happens to be the one manga my wife still keeps up with outside OP.
It is extremely EXTREMELY wishy-washy girly and I don't say that as if it's a bad thing but there really is little there for guys. Since she started OP for my sake I attempted to read Nana MANY times and I still can't get into it. It's just not for me. But I DO like how the author handles the 'passage' of time which brings us to
Nodame Cantabile is the other manga "for the ladies" on the chart which seems to be doing almost as well as Nana. And it's THE SHIT, greg.
I agree about Nana, and I like Shojo. Aside from a shojo actually having distinctive and well done art, I really don't see the big deal about Nana and have tried once or twice to get into it. I know a few guys who're into it and sing it's praises and tons of girls who were pretty adamant about it being "the best thing ever!!!", but it just didn't stand out to me at all. Even the music in the anime version was pretty underwhelming.
Nodame on the other hand, is Josei, so the whole thing just felt way more mature romance-wise. And for once there is a series writte by a woman where there arent any magical, scenes with fairies playing and handsome guys just fall into the ditzy, plain looking girl's arms and would immediately die for her, just because. It also helps that the eponymous main character, Nodame is one of the single most entertaining and unique characters I've seen in years and the the two leads have genuine chemistry when they're together. They play off each other really well, and even if you're not interested in the romance, it's pretty much a full-fledged classical music series which approaches that the way sports manga do and does a solid job of making that whole spectrum of the series interesting!
I guess it's obviously not for everyone, but if you and your wife are looking for something girlier to bond over, I'd recommend trying that out. I've been pretty gay for that series since the anime started in the beginning of 07.