There are several reasons why koi kaze may not appeal to someone. The most obvious being its subject matter: an incestuous love developing between a 27 year old man and his 15 year old sister. Another is the slow rate at which the story progresses, due in part to frequent lengthy scenes where little is said and less done. Some may find the main character, koshiro, dislikeable. However, those who watch this anime with an open mind could well find a lot more right with it than they would have thought.
The anime deals with a taboo topic* in an utterly non-judgemental manner. The characters are portrayed as conflicted, unhappy, indecisive, inconsiderate; in a word, human. I can think of very few television programs of any kind with characters portrayed in such a believable manner. They find themselves in an extremely unreasonable situation, and they make mistakes, have regrets and do things you, as a viewer, will want to shout at them for. But as Niel Gaiman's anthropomorphic Death said, its hard to hate someone you know so well. I should mention that when I say "characters", the only two I mean are the siblings: koshiro and nanoka. All of the other characters exist primarily as a device to show how the main characters interact with the world outside of themselves.
As I said before, the anime is rather slow moving. People used to shows where epic fights between ninjas and giant robots break out every five minutes or so might find it tedious. This is purely speculation though, as I found the pacing to be fantastic. So many shows seem so intent to tell you as much story as possible that they forget all about mood. The story of koi kaze could probably be compressed down to a 4 episode OVA easily enough, but it would certainly not be so wonderfully told.
The music also is a huge boon to the atmosphere of the show, when present. Music is used infrequently, but when it is, it never fails to add appropriately to the scene. As all good BGMs should be, it is pleasant but unobtrusive. The animation itself is simple, but expressive. It uses a palette of mainly pastels, unlike the bright, often garish colours used in most anime, which suits it perfectly. While not the most visually impressive or memorable anime I've seen, I find its style hard to fault. Much like the other aspects of the show, it takes a minimalist approach, and it works brilliantly.
When reviewing anything, I'm always hesitant to give a perfect score. In this case though, I can think of little that could be improved. While I'm sure there would be many to whom this not appeal very greatly, patient, mature and open minded viewers will find a lot to appreciate. If you feel those are attributes you posses, or are just looking for something different, I would recommend koi kaze absolutely and wholeheartedly.
* Funnily enough, while the main plot of the anime involves an incestuous relationship with an under-age girl, one of the episodes, which dealt with the siblings' parent's divorce, was not broadcast on Japanese television due to divorce being viewed as a taboo subject. I love Japan...
