This is just something I've noticed over the years - the anime industry goes through cycles of popular seiyuu. Sometimes I notice that a whole slew of new series feature the same few VA's for the main roles.

Right now the popular ones seem to be Hanazawa Kana (Zange from Kannagi), Sawashiro Miyuki (Canaan from Canaan) and Toyosaki Aki (Yui from K-on).
A little while ago it was Tomatsu Haruka (Nagi from Kannagi) and Inoue Marina (Kana from Minami-ke).
A bit further back was Kugimiya Rie (Shana) and Hirano Aya (Haruhi).

I'm not sure how many others here pay attention to the voice casts. Does the voice cast really make that big of a difference in the success of a series?