I've only read 16 chapters so far, but Boukyaku no Shirushi has an extremely endearing premise. It hasn't exactly been "never seen before". There are lots of mangas out there with political marriages, or a king finally getting married after many failures. The thing with this though is that nothing is overdone. The plot is simple, the obstacles aren't grand- but it is what's needed in a light-hearted and at times comedic story such as this. And I mean come on, isn't the premise of a "headless king" interesting?
It's not a completely unique art style, but it's still done well in my opinion.
The characters are drawn cutely, and they are pleasing to the eye.
The characters are simple to understand- their personalities are rather predictable. However, there's nothing really wrong with that in this manga as they are all so lovable. They don't get annoying *mostly*, and at times the simplicity is what makes them so endearing. The relationship of Lilia and the King is also lovely.
I'm going to try to not give away any spoilers, but I really enjoyed this manga. It's not one of my favourite shoujo mangas, but it's still good. I particularly like the fact that the king is "Headless". Most of the characters have their own charm that, while have been seen before, still remains unique. to them. There's just something about Lilia too that makes you want to root for her success in her relationship with the king.
This is just my opinion though. I'm more than a bit of a softie for fantasy-setting shoujo manga.