To start with, that's one of the things I liked: there's stuff to give away. Most good dramas have some kind of revelation of previously unknown information, but Noir as a series is sort of like a single long, almost continuous gradual revelation. It think that some people might think it's kind of slow in pacing (I felt that way a little bit in the first few episodes), but really it's much more deliberate than slow. It gives up its secrets in little bits and pieces, and it's a lot of fun to piece them together and theorize about what's "really" going on as you watch the series. I almost want to use the word "tantalizing" to describe how the series tells its story, but I worry that that would be a little pretentious, so I will err on the side of caution and refrain.
[Except that I just *did* use the word! Hooray for paralepsis! Hooray for both having and eating cake!]
Another thing that I liked about the series is that it gives the veiwer a lot of credit for being able to figure things out. Too many shows assume that the audience is dumb as a box of hammers and try to spell everything out. Feh, I say! I'm smart, and I like figuring it out myself, so I really appreciate it when a show lets me do that. The events of the series are pretty self-explanatory, but there's a lot of territory in the realm of character motivation where they leave things unspoken.
But you can still figure it out, because they do a really good job of building character depth. I was really surprised at how much emotion and development they manage to communicate just with facial expressions and the like. We watched an interview with the voice actresses on the last disc (which was both interesting and annoying) and one of the things they said was that they'd have to do 25 takes on just a sigh or a gasp, because these little noises had to convey a lot of complex emotion. They did a really good job with that.
Characterization was also very strong. There were a number of times (especially in the last few episodes) where thing happened in sort of surprising ways, but then you think about it and say "well, of course. Given who those characters are, it really couldn't have happened any other way". I think I like it when a story has something unexpected happen and you feel like you should have seen it coming, because in retrospect it's totally consistent with everything that came before.
Let's see... I think I already talked about the fact that it was good at surprising me, and that I liked that. But a story has to surprise in a natural and organic way, so that everything still fits together.
I think that really gets to the heart of what I liked about Noir: I really like it when a story (series, show, movie, book, rpg, whatever) has a structure that is both complex and coherent. Where it's rich enough that you can't really predict what it's going to do, but as it unfolds you can see how all the pieces reflect and relate to one another in a way that is... elegant, I guess. A story that is interesting and meaningful from many different perspectives. And Noir did a pretty good job of that.
Basically, it made me glad I spent my time watching it. That's really what counts, right?