Categories/Tags are hard to get right. One problem faced by the Bukkit mod, which adds a plugin interface for minecraft servers, is that the plugin authors would tag everything that would even remotely apply to their plugin. I might be browsing for an economy provider plugin in the economy but the category list will be polluted with shop plugins listed (which does make sense) to land management plugins like Towny just because there is an option to buy land. Spaceport's categories seem very part centric. I haven't uploaded a mod to spaceport but I assume the author selects which categories/subcategories of parts the mod applies too. I think this works well for the use case of someone browsing for a specific part type ("None of these engines are working for me which mods have more engines..."). It doesn't seem to serve well for organizing mods by themes (saturn v, space x, etc) or mods around a mechanic (kethane, interstellar, remote tech). There are multiple use cases that are some combination of: browsing for mods but not looking at something specific, browsing for a part type, browsing for a mechanic, browsing for a theme, searching for one of those categories (mods that offer reentry, parts that are apollo mission themed), and searching for a specific mod. One solution is to offer a small list of predefined categories/tags where the mod author can only pick one or two tags that best apply for their mod.