I thought the first episode of K-On was brilliantly funny. And, of course, deeply moe. And the voices were wonderful. They were hilarious, with some great timing and expressiveness.
Kyoto Animation seems to have the knack of picking out lesser-known voice actresses who can really do the job for them: Hirano Aya, Gotou Yuuko, and Chihara Minori in 2006 in Suzumiya Haruhi; Katou Emiri, Endou Aya, and Fukuhara Kaori in 2007 in Lucky Star.
In K-On, except for Toyosaki Aki (Yui), the three other main voices are relative unknowns. (Click images to enlarge.)
Satou Satomi 佐藤聡美 (Ritsu) is 22, and works for a major agency, Aoni Pro. But she has had few roles before this, notably characters called Yaya in Labyrinth and Wakana in Shugo Chara. Here is her blog.
She has clearly studied Orikasa Fumiko playing Miu in Ichigo Mashimaro. You could say parts of the performance are derivative, but if you're going to imitate anything, OriFumi's great comic performance is a good thing to choose. Satou-san is so far making the outlandish character work. Maybe making the character outlandish is one reason it does work.
Hikasa Youko 日笠陽子 (Mio) is also with a big agency, I"m Enterprise, and is 23. Her only previous role of any note was a character called Minamo in Shetchbook.
She doesn't have as wild a character to do as Ritsu's, but the naturalness and energy in her voice give life to the quiet character. Her is her blog.
Kotobuki Minako 寿美菜子 (Tsumugi), like Toyosaki Aki (and Tomatsu Haruka and Takagaki Ayahi, as well) is with Sony's Music Ray'n agency. She has done a character called Hijiri in Kyouran Kazoku and not much more. It's understandable that she has done so few roles, however, since she is only 17.
I like the little break that comes into her voice when it rises from passive ojousama to sudden enthusiasm. And there's a dry "oh, good" toward the end of the episode that makes you think she has talent. Her blog.
Toyosaki Aki 豊崎愛生 (Yui) is 22 and has already had some good roles: she burst onto the scene as the irrepressible nude swimmer Amuro in Umisho, and has been Su in Shugo Chara, Yoshino in Minami-ke, and Najimi in Akikan.
She and Kotobuki-san, with Tomatsu and Takagaki -- the four have been developed together over several years as seiyuus and singers by Sony's Music Ray'n -- will release their first single as Sphere on April 22, the OP to Hatsukoi Limited.
I think Toyosaki-san is just excellent in episode one. Her sa ("not sure") when asked what the Light Music Club did, was brilliant. Her performance repeatedly gets your attention. Her blog.
I also think some credit must be due to the sound director, Tsuruoka Yota, who was the sound director of Lucky Star and Suzumiya Haruhi, as well as of such other well-voiced shows as Turn A Gundam, Blue Drop, and Rozen Maiden. Especially with the director being an animator doing her first directorial job, I suspect the sound director had a lot to do with how the lines were delivered.
It's amazing that the voice-acting seems to be working out so well, since they selected the seiyuus in part on the basis of what instruments they could play. The ED is played and sung by the four seiyuus. EDIT: I now realize this isn't true. The seiyuus are just learning their instruments. Maybe they will have learned well enough by the end of the series to perform.
EDIT 2: More characters
Nodoka: Fujitou Chika 藤東知夏
Yui's friend Nodoka is played by Fujitou Chika, in her debut role. She is from the seiyuu/talent advanced class at the Avex Artist Academy. She has her own blog. Here is her Japanese Wikipedia page. And here is her ANN page (name currently misspelled).
Ui: Yonezawa Madoka 米澤円
Yui's sister is played by Yonezawa Madoka, who is with the big 81 Produce agency, but has had only three roles over the years since 2006 -- until this year, when she is playing this role, Lashara Aasu XXVIII in Isekai, and Kaho in Gokujō!! Mecha Mote Iinchō. Her agency page says she sings, dances, and paints. ja.Wikipedia. ANN. personal blog.
Sawako-sensei: Sanada Asami 真田アサミ
Sanada Asami is the biggest name among al the seiyuu in this show. She plays Sawa-chan. Her debut role of Di Gi Charat was her biggest, but she has also been Jun in Rozen Maiden, Tamaki in Kurenai, and most recently Kanako in Maria Holic. home page. ANN.
Azusa: Taketatsu Ayana 竹達彩奈
Twenty-year-old Taketatsu Ayana plays twintail Azusa, the freshman who joins the club in episode 8. She has made a name for herself as Ako, one of the sisters in the raunchy incest comedy kissxsis. She also played a baby in Marimite 4. Those are her only two previous roles. But I don't think they will be her last. She's with I"m Entertainment, and lists her interests as karaoke, reading, shopping, and tennis. home page. ANN.