It's such a pity that there are so few comments here. I guess most readers, after reading this manga, were in lack of words. They cannot describe, or reason how they were so profoundly moved and impressed by the manga.
I bet those who haven't read the manga also think the same thing. On the surface, there seems to be nothing special about it, no steamy sex scenes, no handsome guys, no beautiful art, no melodramatic plot. So why does it receive such an impressive rating?
I have read hundreds, may be thousands of BL manga, and I can confirm that Hydra is truly a gem in the Yaoi, maybe manga world. It is realistic: the author didn't attempt to idealize the characters, they appear more human than most characters in manga nowadays. They're so real that you can imagine them existing in real life, being a part of your life!
Her characters are not perfect. They have flaws, like any of us do. Ryuu, the seme, is not intelligent, nor rich, nor powerful like the all famous Asami of Viewfinder. In fact, he can be just any ordinary man you see out there, if it wasn't for his love towards Hitomi.
Hitomi is not beautiful. He doesn't have sparkling humongous eyes that may have taken up half of his face like those of girls or other ukes; he has a man's body frame. He is not a woman and he certainly doesn't look like one. He is not the typical stereotyped uke that you expect.
The plot itself is also not over the top. It doesn't involve love at first sight, and there was not any death, nor cancer, nor memory lost nor anything that you may find in Korean melodrama.
Nevertheless, all of these compound into a masterpiece. The striking quality of the manga is how it makes the love story so realistic, and yet doesn't turn it into something uneventful and tedious. It enables the readers to empathize with the characters instead of idolizing them. I cannot recount the number of times I have cried for the protagonists after rereading the manga over and over again.
I understand that the manga may seem dull and boring for those who have only read a few titles. Don't be quick to dismiss the yaoi's success, please continue to read up some more BL novels, and return to read this manga when you've grown tired of them. That will be when you realize the ingenuity of Miyamoto Kano in crafting this manga.
For those who are hooked, please don't hesitate to read her other works. I recommend Rules and Not/Love which are enjoyable and poignant at the same time.