This community-created, officially endorsed guide to defense is a useful primer for newcomers to Arma's hard-sim approach to modern military combat, but doubles as a good showcase of Arma 3's strengths.
Arma 3's scale and realistic understanding of the effective range of modern weaponry creates deep battlefields. And I mean physical depth, measured from your position to the point where you have to start engaging the enemy. Scouting tactics are vital when the enemy is kilometres away. Arma fights happen on a scale most RTS games don't simulate.
It's an ambitious, impressive co-op game—one of the best—but it demands plenty of time and patience. Not that there's anything wrong with that, of course, it just means that videos like these are particularly helpful. Here's another one on machine gun teams.