If you have been diligently adding your keywords, captions, location information, and other metadata using Aperture (or Lightroom, or Photo Mechanic, or any other photo browser for that matter) as you know it can be very useful for finding images fast. Even if you haven't been so disciplined adding your own metadata there are still some interesting and fun things you can do with Aperture on a crappy Saturday afternoon if you are not inspired to go out and shoot.
Let's assume that you have not been that diligent with the keywords. There is plenty you can do and learn from the EXIF assuming you are shooting a digital camera. I love not having to take notes anymore. Not only that but all of that is in a database. Here are a couple of ideas that I use every once in a while to explore my photographs, my habits, and preferences over the long term using EXIF focal length and lens model data.
I love to search my entire library using the EXIF focal length field. Actually I use the 35mm equivalent field since I shoot with two different sized sensors. Typically I will use the range option in the search HUD so that I pickup all of the shots that are close to what I am looking for. As a for instance using a "in the range" search between 17 and 21 on the "Focal length (35mm) EXIF field will give me all of the ultra-wide shots used on both my DX and FX bodies as well as all of the lenses that I happen to have used no matter whether they were fixed or zoom.
I find it incredibly instructional to understand my shooting habits and preferences for a lot of different reasons, especially now that I use a lot of zooms in day to day work. I can easily tell you what lenses I use constantly, but I would have a hard time telling you what focal length regions they happen to be set at. You can use this in a lot of different ways in combination with the other EXIF like shutter speeds, apertures, as well as subjective evaluations of the images shot. For instance, I wrote about rethinking Nikon's new 50mm AFS f1.4 lens a while back. One of the things that spurred me to do this was searching my library for "Focal length (35) in the range 40 to 60 and the range 70 to 85. What I found was astonishing.
If you would have asked me if I use a "normal" focal length I would have said that I do but I found that on both my DX and FX bodies I use it way more than I thought I did and an unusually large percentage of my higher rated shots happen at that focal lenth. Almost an equal number of images shows up between 70 and 85 on both DX and FX formats. What's more is a quick look at Aperture's shows me at F2.8 a lot on lenses that happen to have and F2.8 maximum aperture. As I evaluated the images shot at F2.8 it became appearent to me that the choise of that aperture was not only due to shooting conditions but an equal number of times for aesthetic reasons.
Wow, I have an old AF 50 F1.4 but I wasn't using it on either FX or DX - after careful thought and consideration the obvious answer was because it was more flexible to not have to change out lenses. Looking at all of the situations I could definitely see that that was true in some of them but more often than I would have guessed that focal length at normal on FX or short tele on DX would have been great for the entire shooting session. The real reason I never put the old 50 on any of my current bodies was that it was not AFS making shifting between auto and manual focus a pain, making auto focus slow, and having a horrible touchy iffy feel to manual focus. I ordered an AFS 50 F1.4 that week. I imagine when I do a search down the road that lens will see an amazing amount of use on both FX and DX bodies.
If you happen to go the other way - say slightly wide to normal the new AFS 35 may be a better decision. In any case learning preferences, actual habits, aesthetics, etc by using searches on your image library is a great way to do learn things about your photography and guide both purchasing decisions and behavior. The reason I will probably not go with the AFS 35 myself is because I shoot FX and DX, the 35 is DX only. After thinking about that for a bit and the number of "normal" focal length images shot on DX that were for low light situations I decided that I will probably be shooting those on FX going forward and the 50 was the better decision for me. If I shot a D200 or D300 or any DX body exclusively I would by the 35 AFS in a heartbeat.
From a behavior stand point I can also show you what lenses and focal lengths you do not use much. This could be due to subject matter, due to taste, or due to habit and a weakness of vision at a particular focal length. For those focal lengths I like to assign myself mini-projects to shoot at that focal length (or range) for an extended period of time without resorting to ways that I happen to "see" a subject. I find that doing this is a great learning and growth experience. I rarely find that it is the subject matter that is the issue. More often than not is sort of "writer's block" and lack of ability to visualize a subject through at a particular perspective.
I call this using your metadata proactively, prior to Aperture, Lightroom, Photo Mechanic, iView, etc. I used metadata, or notes, whatever you want to call it much more reactively when questioned about details of a particular image - I would look it up. Using your metadata proactively as a learning or a creativity tool or just to weigh future purchases focuses more on overall shooting habits and behavior than it does details about a single image. In other words looking at all of the data first and images that have that data in common vs. looking at the image and then looking at the data behind it.
If anyone found this remotely useful I would be happy to let you know some of the wacky things I do to generate ideas with keywords. Just post a comment and let me know.