Habitual non-Acceptance
As hopefully almost all users are aware, almost all those who answer on TSE main site are trying to (and usually succeeding in!) providing a service that is free, quick and for which many alternatives are not free, or are free but of lower quality. Those kind enough to offer answers are sacrificing at least their time to help others, for nothing of value in exchange. Even a Comment such as "Thank you, just what I wanted!" is rare (and likely to be deleted, at least on some SE sites).
So I consider it very bad manners to fail on a large scale to accept answers. (I excuse occasional users unfamiliar with the conventions here.) Acceptance of a bad answer is worse (IMO) but when a user knows about the green tick and makes a habit of withholding it without even explaining why a suggested answer does not suit, I am as peeved as I am by all bad manners.