First things first: This series contains some shounen-ai/BL. Don't like, don't read.
Now that that is over with...

Plot/Story
The mystery that drives Loveless in the beginning is at first a pretty classic Whodunit?, basically, who killed Ritsuka's older brother Seimei? However, as answers are found, more questions continue to pop up. Loveless's story is very intriguing, often riveting. However, the plot tends to move a little slowly; sometimes a single day is contained in one whole volume. There is also sometimes the occasional filler. Regardless, I never got bored reading Loveless.

Characters

Probably the best thing about this manga is the characters. Soubi, Seimei's old Fighter unit and Ritsuka's new one, is probably the most interesting individual I've ever seen, hopping from one mood to another at strange intervals, (as Ritsuka says, "How could he mention murder and love in the same breath?"), and often displaying somewhat skewered morals, ("You have to kick him while he's down.").
Even comic relief characters have depth in this drama. Yes, even the much hated Yuiko. While indeed a pure-hearted idiot, she is also capable of interpreting people to a highly sophisticated degree. The sadistic Zeroes also show a degree of compassion, and rather campy Kio displays courage in the face of adversity.
Yeah, the three-dimensional character designs and the changing character relationships are most certainly the best part of this series.

Drawing Style

I'd never read any of Yun Kouga's work prior to this series, but I immediately found the art stunning. It changes over time, becoming a little more minimalist, but it is still beautiful to look at. The most iconic aspect of this series' style is the neko ears that some of the characters sport. But these have a point, which is to add to the underlying theme of sexuality throughout the manga, (they lose the cat ears when they lose their virginity). I never liked the cat-girl/cat-boy fashion very much, and it was one of the reasons why I stayed away from this series for so long. But they blend in so well with the hair, that after a while you don't even notice them anymore, it just becomes an instinctive part of the character.

Enjoyment

This isn't exactly a thrill ride to read, nor is it a chuckle fest. In fact, a lot of the time, it is downright dark and depressing. Thus, I can't say that I always ENJOYED reading it. Regardless, I was never able to stop reading it either.
If you are sensitive to violence, I would suggest not reading this, or at least skipping a few key scenes, (the part where Soubi gets a nail through his hand, for instance... and most parts where we see Soubi's past...).

Overall

This is one of my top three manga series, usually placing at second. I am always eagerly awaiting scanlation releases and new English volumes. I hope you enjoy it too.