You might like peculiarity. OED provides the definition
1. The quality of being peculiar to or characteristic of an individual person or thing. Also: that which is peculiar to an individual person
or thing; a distinguishing or special characteristic.
While this definition suggests that the word is only applicable to individuals, which might rule it out from being applied to (all) Americans, several of the citations use it to describe the quirks of groups, and I've certainly never avoided applying it to groups in many years of native (BrE) speaking.
Under peculiar OED provides
5. gen. Singular, unusual, strange, odd.
which I think justifies the use of peculiarity for OP's requirements.
As to whether this is "negative enough" I'm not sure. I think it might well express the pot calling the kettle black, that is your habits are peculiar, mine are entirely normal.