Fun fact: 9/10 users with no linked gamertag are better than you at Halo. That doesn't necessarily mean that I am; but it does mean there's a pretty solid chance.
Running in straight lines when under fire.
Using Armour Lock. Yes, there are 'experienced' players that do this too, but nine times out of ten they're on the same level as kids who've just picked up their first controller.
Dropping their DMR/Magnum for a power weapon instead of their AR.
Running around with a Sword or Gravity Hammer out.
Standing at the top of Grav Lifts.
Repeatedly running up the same Grav Lift.
Developing a vendetta against a specific player, and continually rushing said player despite the fact that they're getting wrecked every time.
Double beatdowns. 'nuff said.
Running around the inside of a tight corner.
There are more, but that's what I've got off the top of my head.