Describing what makes UN-GO special is hard, but I'm going to take a shot at it anyways to add to my two brief earlier attempts.

On the surface, UN-GO is a show about a detective solving mysteries with the help of his magical assistant. There are many ways that this could go wrong, but UN-GO avoids them all; it does essentially everything right. In the process the show is not so much about the mysteries as about everything that is going on around them, about the characters and the overall situation and the background. The recurring characters and the ongoing situation are both interesting enough to support this. Everything is interesting and multifaceted, and the show is not afraid to use a light, indirect touch to illuminate things. Above all, I felt that the show was plain smart; it was intelligently written and presented intelligent, multifaceted situations.

(One disclaimer: this is not a puzzle show. The mysteries are not necessarily intricately constructed and while the show does often foreshadow the solution, it doesn't always give the audience enough clues for us to come up with all of the answers ahead of time.)

Part of the pleasure of UN-GO is that it is not direct in the same way that many other shows are. For example, there are some unpleasant and creepy people in UN-GO but the show is by and large devoid of the stereotypical ways of showing this; instead it lets these people talk (and has some of their actions come to light) and then leaves us to draw our own conclusions about them. I've read a description that calls it a mature show, and I agree with the label; it's a low key, grown up show for adults that's appealing in a more subtle way than the usual anime fare.

To repeat myself: note that UN-GO is not necessarily a show to watch if you want to see actual justice happen. The setting has a quietly totalitarian government that is a strong believer in 'realpolitik' and quite often the government covers up the actual crimes with politically expedient false explanations and thus lets the real perpetrators go. This isn't presented as a good thing but at the same time the whole system doesn't wind up going down in flames; any successes that the protagonist scores against the system are limited.

(Also, as mentioned the show contains some supernatural elements.)

After seeing the ending of the show, I'm now not certain that I want a second season. The show ends at a very good point but it is a pivot point; a good second season to have to be very different than the first season.

(Explaining this requires both semi-spoilers and a separate entry.)

On a side note I feel that UN-GO exactly the sort of unusual but very good show that justifies noitaminA. It's not commercial in the conventional sense and I suspect that it wouldn't have been made but for the existence of the noitaminA block.

Liked: very much. I feel that this is an excellent show.
Rewatch: possibly (I'm not strong on rewatches, but this is a good candidate to get more from on a second viewing).

Other reviews or commentary: chaostangent, The Cart Driver, GAR GAR Stegosaurus, metanorn. Reviews may contain spoilers.