[ghaff]

I suspect you're looking for a more absolutist set of principles than exist when, in fact, these things exist on a continuum and opinions vary. I think there are certain overall factors that trigger the broad McMansion dislike but they're not universal.

(From an architectural perspective) a lot of mishmashing of architectural styles and features without any sort of unifying aesthetic. Yes, that's a matter of taste, and there are ugly buildings with unified modernist aesthetics but we can probably agree that architecture should be more than pich two from column A, one from column B, and one from column C.

In many cases, McMansions are in large developments with big houses on small property lots with, often, minimal landscaping both for reasons of space and because they're fairly new, which accentuates all that there is to not like about the houses themselves.

I suspect that the McMansion style gets wrapped up in a lot of cultural biases and attitudes such as you see here.

[ADDED: The OP did make an effort to offer some tests. My untrained paraphrase would be that McMansions tend to be sort of a jumble of often seemingly random design elements.]