I would push Sawashiro Miyuki back to the same set as Tomatsu Haruka and Inoue Marina.

Fujimura Ayumi has been in a great number of series in the past three seasons, and a number of them leading roles. (Mafuyu in Qwaser, Cecily Cambell in Sacred Blacksmith, Niche in Letter Bee, bit parts in Ladies versus Butlers! and Hanamaru Kindergarten, and now Misaki in Kaichou wa Maid-sama) She even played three different roles across Nodame.

Kobayashi Yuu has also had a massive resurgence in popularity. Rio in Sora no Woto, Candy-girl in Bund, Kanon in Umineko, Nagi in Nyan Koi, Hiroshi in Ookami Kakushi, Mariya from Maria+Holic. It helps that she can play male and female roles though.


Producers will push strongly for popular VAs to take lead roles. Often most of the cast will be imported from the Drama CDs (in manga adaptations) with the leads replaced by more well known (or currently popular) ones. It also helps on how much sway the voice company a particular VA works for has. The bigger ones can "bump" actors and actresses from smaller companies.

edit:

Alhuin is right, it is all about the audience draw. Whoever is "hot" at the moment will get a lot of roles. But it really helps on which company they belong to.