You can open doors at any angle by using the left control stick,
use this to keep your guard up when entering rooms.
You cannot reload when holding doors.
You can kick doors open by pushing the melee button (PS4: R3, PC: left stick button), or just by sprinting into them.
Doors will make a sound when opened, so it's good to get into the habit of closing doors you pass through to alert you
if anyone tries creeping up behind.
Doors that are held cannot be opened from the other side.
You can attack players that are holding doors using the gap at the bottom, crawl down and shoot, or lob a grenade.
Please see the following for more info