Of the d20 games I've played, I found the system, with all its fiddliness and customization and unified mechanic, worked best in d20 Modern (and d20 Future). I've got a few ideas on why it actually works.
A) It's designed to work without magic, and it does.
B) Having all sorts of skills and abilities mapped out seems to work well for emulating modern to future settings better than a strict class system.
C) The base classes, modeled to fit each ability score, are iconic enough to be classes, but are used more like ability templates. It's really easy to create the sort of character you like.
NPC - Groth
2 hours ago