Mechanical complexity is something very few reviews take into account. This is completely understandable - most people don't know how much work goes into making some kinds of mods. Even those equipped to judge are usually too specialized to do so - for example, I can look at a mod and say "boy, that's an intricate set of scripts right there!" but I have no idea if a mesh took 30 seconds or 30 days to make up.
And really it doesn't matter to the end user anyway. Just a pet peeve of mine.
Anyway, something to note would be the impact the mod has on the engine. Does it make the game a little less stable? More stable? Is it compatible with few, some, many, or most other mods?
Are you measuring mods relative to other mods? Or just based on subjective analysis? And are you going to give ratings, and if so, what kind of rating (3/5, 72/100, bad-okay-good, etcetera)?
And... what sort of project is it? Are you starting up a website where you review mods, or a blog here on LL, or something else?
EDIT: And for which game?