Kamisama Hajimemashita was the first anime I watched, besides Studio Ghibli movies, and the first manga I read. What stands out in my mind in terms of her style is her nice use of screentones and her many beautiful floral scenes.
Something else I enjoy about this manga is the storytelling and characters. Nanami is a strong character who follows her own feelings and doesn't let herself be influenced by others, meeting the unusual events she faces head on, but she is still just a teenager who faces the same challenges and problems as regular teenagers.
So far I have only read Tsubasa: Reservoir Chronicles and X/1999, but I am in love with Clamp's art style. Gorgeous flowing lines, intricate backgrounds and compositions, unique and instantly recognisable character style. I only wish I could one day produce something so beautiful...
Doubling is a theme which crops up again and again. I always find works dealing with this subject interesting.
I have heard that there is quite a lot of diversity in the style of Clamps different works, so I want to read more of their manga series to compare them. I have also read that the clamp artists are largely self taught, which gives me hope for the future!