Why's it unexpected? Well, although it may start off as pure, camp shoujo-ness in the first volume (as the previous commenter pointed out), Eroica swiftly evolves into a genre-busting manga that appeals to readers of all kinds. It's a gripping, fast-paced, action-oriented story with amazing plots and hilarious humor. In later volumes, the art becomes very masculine (in a good way!), and the camp is much more mature. Dorian's homosexuality is matter-of-fact, and somehow manages to be understated and glamorous all at once. Not that his character is understated in any way! Good god, no. But the mangaka's portrayal of his character is very clever and subtle. In most BL stories, a protagonist's sexuality is the centerpiece of his character -- but for Dorian, there's a lot more at stake. He's a criminal, an aristocrat and an aesthete -- and there are layers to his character that make him palpably real, rather than just That Gay Guy.
In fact, if I had to recommend BL to a non-BL reader, I'd start them off with this. Sure, the title character's gay, but there's just so much more to the story than that! The plot's based on politics and espionage and thievery, rather than on smut. Which actually makes this a more emotionally realistic story -- you don't just have one mindless sex scene after another, with the two protagonists jumping into the sack as soon as they get the chance. No. This manga's not about the sex (which hasn't even happened after 30 volumes!). It's about the chase, in every sense of the word -- the chase of art, of beauty, of information, of power, and of the human heart. It's just awesome. I feel satisfied after reading every volume, as if I've had a meal that fulfills all my requirements of taste and nutrition.
Oh, and. Since the plot is formed of interconnected stories, you don't necessarily have to start at volume 1 and read every single volume along the way. If the thought of catching up with so many volumes freaks you out, just read the most recent releases and see if they're to your taste. If they are, then you can go all the way back and embark on your journey into the Eroica fandom -- a land of no return, I promise you, if only because that manga's that. Damn. GOOD.