I learned quite a bit. First off, I learned a lot of Japanese just from reading manga and watching the anime. Just basics, like the honorifics, basic greeting, etc. etc.
I also learned a lot about deduction (which is why I'm taking a mystery/suspense literature and film course, so excited) and how to pay attention to the little details. It helps that we did a CSI unit in Science as well . Almost everything shown in DC has something to do with the murder, so I suppose it helped my memory as well. Nowadays, I notice a whole lot more than I used to, including license plates, hair color, etc. etc. I can deduce to some extent and have a relatively good sense of intuition.
DC definitely helped my writing. It gave me a whole lot more inspiration after a time when I rarely wrote stories anymore, you can see my progress at Fanfiction.net.
DC also taught me a lot about anatomy, and I will never forget that lesson from Hattori about the state of the corpse and how it can be used to determine how long the person has been dead. The lesson about luminol was interesting as well, and I now know that copper coins can reveal traces of poison.
I've also been taught to think outside of the box, to not accept everything as it appears to be. In several cases, the one most notable to me being the case where this golfer died in an explosion, the most obvious answer was incorrect. There's often a hidden meaning just below the surface and a lot can be discovered if you don't focus on one tiny aspect of it.
There's a pattern in DC too...usually the one who you think looks the least suspicious is the murderer. When I was younger I jokingly deduced that way: "That lady seems really nice and sweet, not to mention innocent looking...she's the murderer." Guess what? I was right.
After the first 100 or so cases, my mind began to take in the details and deduce the mystery for myself. I took great relish in it, notably when I guessed exactly what KID was doing before Conan did in the "teleportation trick".
Of course, there's a lot more that I could add to this, but these are the main points.