As isteveus said - the loop browser in GB has its own categories.
If you are not happy with the way Apple indexed a loop, you can save a new copy (Edit > Add to Loop Library) and pick your own tags or change the name to make it more descriptive.
To have a database of your finished GarageBand songs, share the finished songs to iTunes - this will give you all kinds of search criteria to retrieve the songs.
But you are right - there is really no database to retrieve GarageBand projects. I use essentially the Finder and a set of folders to structure my hundreds of finished and unfinished projects, plus Filemaker '11 to remember where I have the sheet music, midi files, song lyrics, etc. But I do not know of any ready made Filemaker template. I simply used "define database" to set up my own database view.
Send feedback to Apple, to let them know that you want GarageBand to have a project database like the other iLife applications: http://www.apple.com/feedback/garageband.html
Here's another thing that comes to mind: unfortunately, iTunes doesn't open .caf files (at least not on this computer that I'm trying it on), but you can open the loops (from HD/Library/Audio/Apple Loops) in an audio editor, save them as .aiff files and put them into iTunes. Then you have all of iTunes' database capabilities to tag them, sort them, preview them etc.
I've used Filemaker for years and thought it would be fantastic to have two such powerful tools GarageBand and Filemaker working together. There are times I'd love to be able to make a more elaborate search like "Toby Keith>Courtesy of the Red White and Blue>Loop 4 Lost eye"!
There are just soooo many things you can do with information within a database. If I build one I'll make it available to everyone here. The first thing I'd have to accomplish is to make it seamless between the two products, lets see how far I get.