I carry a notebook and pen around with me everywhere I go for a variety of reasons, one of which is noting what I like and don't like in the design of the world around me. More often than not it's what I don't like that gets penned down, though I'm not sure what that says about me.
I started this habit when I was a journalist, and it's stayed with me ever since. I usually use the Moleskin Pocket Notebook. It's a bit more expensive than some, but holds up well and is quite comfortable in a front pocket.