People have several reasons for liking Clover. These are a few of mine.

The art is stunning. Suu, Oruha, and Lan are so beautiful and graceful and Gingetsu and Kazuhiko are very handsome. The layout is unusual and the effect is profound. Fairy Park is a awesome looking place and a lot of the scenery is just... wow.

The story is sad but not tragic. It is subtle and powerful at the same time. The ending fits the story perfectly.

The characters are great. There is no possible way to avoid liking at least one of them. (I like all of them personally.) They are vivid and very charming. Suu can look stoic, sad, and thoughful in the same picture.

It has a little bit of romance, yaoi, action, and magic mixed in together with a hint of the Clamp flare for twisted events.

The songs that accompany the story fit it very well. They are wonderful to read alone, but they accent the story perfectly.