It is because the characters feel real to me. Now Fumi Yoshinaga is the creator of the series Antique Bakery which wasn't a shounen-ai but it had the default gay man who wasn't the main character and his slightly clueless but completely adorable love interest. The next book I picked up was Flower of Life which -wasn't- a shounen ai and didn't have any gay people in it at all (though there was alot of throwing around of the terms 'yaoi' and yuri' because of the fact that it centers around a manga club.
THEN I read "Gerad and Jacques" both volume 1 and 2.
I've no words to describe it. There was a definite plot to it, alot of really awesome plot, the characters were likeable and the first volume made me crawl the walls to get the second volume. (not to mention the character Gerad writes YURI PORN in the second volume.. 12 volumes of it.. teehee. I love my yaoi with a side of hot girl on girl action. Yessireee..)
And now Ichigenme... The First Class is Civil Law. A yaoi.. and how does it start out.. a guy putting his dick in someone's beer and saying that it is human formaldehyde. Okay there is nothing sexy about it yes, but it is DAMN amusing... and it is nice when you can see beyond the seme/uke with her characters.. because she doesn't stereotype their personalities. She just let's them shine through, I know I'm going to love this book just like I loved "G&J". And I'm going to actively hunt down anything else she's written. *coughs*
Edit 1: I DID LIKE IT! ALOT! *this mangaka makes me wibble alot*
Edit 2: SAIUNKOKU SEASON TWO FIRST EPISODE IS OUT! *has been having withdrawals*