The distinguishing feature of a microaggression is that it inflicts damage mainly by attrition. That renders the damage invisible, when taking any one isolated incident. It’s a siege tactic; like any offense, though, it ultimately works towards the same defeat, oppression, and hegemony as an outright attack. It’s only when taken in the aggregate that the damage reveals itself.
Most importantly, an invisible attack is hard to rebuff. You certainly feel it, but no one else knows what’s going on, maybe not even the attacker. By consequence, your defenses are lowered because any response looks like escalation, possibly alienating any sympathy and provoking further attack. This feeds into a kind of gaslighting where you’re not sure anything even happened, even as the inflicted damage remains.
That’s why I feel pretty glad the idea of microaggressions is being spread, to mitigate that cycle.