Creative Differences: Why a second season shall never be made (at least, not by Studio Deen). Due to a hand injury, creator Natsuki Takaya was unable to have direct involvement with the anime adaptation. Sadly, she and director Akitaroh Daichi weren't getting along, and the former got a reputation for being a Control Freak. Takaya demanded that big name seiyuu be casted, something Daici normally avoids. Takaya also had issues with Daichi, but sadly, Deen would only let Daichi handle the series, and Daichi has pretty much confirmed that he won't work with Takaya again.
Schedule Slip: The artist, Natsuki Takaya, broke her left hand - her drawing hand - while working on the 8th book, forcing her to take an extended break from the series. This is one of the reasons that the anime ended the way it did, the other reason was the Creative Differences between her and anime director Akitaroh Daichi.
What Could Have Been: Natsuki Takaya said she planned to have Ritsu and Mitsuru develop a relationship but she couldn't find a spot to fit it into the story. By the time she realized the story was still sitting there (and Ritsu hadn't made an appearance after his introduction) the manga was ending.
Though a small mention at the finale gives the impression that it happened off-screen.