I just started experimenting with this in Bridge today (after realizing I might have lost my ACDSee database). If you use CS4 or 5, you can make smart collections. Those are awesome! I selected my SSD folder and had the smart collection look for all the files that have preview in the title. Now all my previews are already in that collection without having had to tag anything, and the bonus is that whenever new kits are added, their previews will automaticly be added to that smart collection! The only problem is that some designers name their preview 'folder', and others have two previews: one named folder and one named (name of kit+)preview. Still trying to figure out what the best way is to solve that: renaming all the folder files or have my smart collection search for all previews and/or all folders, in which case some of the previews would end up double in the collection...
You can make smart collections for all types of stuff you use a lot, like flowers for example. I know you could just do a search for flowers every time, but if you do that a lot, you might as wel make a smart collection that already has them all in place (without having to wait for your search to be complete).
If I'm going through with this, I will definitely tag the colors and themes in my previews using keywords. You can add subcategories to keywords (not to smart collections).
Also important to me is: number of photos for templates, style for alphas (thin, chunky, cursive, chipboard, metal,..), shape of frames (square, rectangular, round) and for some papers I tag them on wood, newspaper print, graph or lined paper, fabric etc. Because I often search for those.
The designer's name I would add to the files metadata.
I'm only just scratching the surface of the possibilities in Bridge and I'm not sure this will be a good fit for me, but I read a lot about it yesterday and it definitely sounds like it could do a good job! Good luck!