I think, there a only two possible ways to solve the categorizing problem.
a ) keep it simple.
Find five or six main categories and assign every game to one (and only one) of those.
+ good overview
o rough classification
- hard to find a field of similar games
Example:
Commander Keen ==> Action
Websites:
Abandonia (current)
b ) be as detailed as possible
Use a scrollbox with dozens of categories. Each game should have a complete(!) classification using multiple categories
+ find similar games based on genre, type, graphics, theme, setting, view, etc.
o more work with classification
- less easy browsing of categories (more like querying a database)
Example:
Commander Keen ==> Action, Arcade, Jump&Run, Sidescrolling, Platform, 2D, 3rd-Person, Shareware
Websites:
MobyGames (List of categories), HotU
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Personally, I prefer the second approach. That is, because you will never ever find the one perfect set of categories. Many games just won't fit into a single category.
What about System Shock?
- is it a 3d shooter? (1st-person shooting, lots of weapons and enemies)
- is it a RPG? (quests, stats and character development)
- is it an adventure? (storyline, inventory, cutscenes)
Also by using game genres as categories, you will miss other characteristics!
Ever tried to find games with ... say a Western theme on Abandonia?
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Whatever approach you decide for: be consequent!
Especially don't litter you current set with yet another category!
Edit: added link to MobyGames category definitions