Rather static output (ASCII, codepages, chemical elements, food databases, keyboard lay-outs, Latin dictionary, BMI calculator, ...) versus variable output per system (font viewer, clipboard viewer, ...).
You could wonder if the number of apps is worth an own category. Can an user be bothered to check all 94 clocks to find the 2 alarm countdown clocks? Editorial choices.
I do introduce a new level of folders when the number of apps in a folder exceeds some limit, or when there are enough apps to justify an own subfolder. For example, my Utilities-folder contains an environment-related subfolder "Geography" (mixed OSes). Route calculators, maps, Cosmos/2, Moon, Night Vision(s), PM Globe, Weather Calculator, Xplanet.
I can image that someone would move the Weather Calculator to your unit conversion-section with variable user input (a wind speed of X MPH always remains a wind speed of Y knots), which would be an understandable editor's choice. At Hobbes it's "misc":
http://hobbes.nmsu.edu/download/pub/os2/apps/misc/wcalcos2.zipI considered reporting PCX2PRN for the Fiannce-category, but it was my editor's choice to not do that. Converting (data conversion) some free downloadable (category internet) image (graphs, graphics) to (typically financial) data. You'd almost wish "tags" would exist. Like "clock", "text mode" and "open sourced", to find all text mode clocks which can be maintained. Cross-links won't work, because a page "text mode" may be too large to be useful.
Following Hobbes will be a good starting point. Unless, for example, an internet directory contains too much (WWW, mail, PM Mail, Mozilla, usenet, downloaders, speed monitors, ...) files to be useful indeed.
It also depends on goals. I'd try to avoid almost empty categories, because that would demote an OS when users are offered one choice. Fore example, I know one internet telephony app and it ain't Facetime (or whatever that's called).