I just posted the first game I've ever published independently here and am psyched to see this conversation. A lot of the most relevant stuff has already been said above, but categories for low prep, no prep, one-shot and campaign would be amazing for folks looking for games to fit specific life situations that come up fairly regularly.
Also, there's pretty big crossover between designers and players of tabletop RPGs and larps, so larp classification tags could also be very useful - things like, well, larp, as well as parlor, freeform, boffer, nordic. I'm not that deep in the classification system, but those are some broad categories that would be a not terrible place to start.