Meta: In a grim, bleak market of cover-based third-person-shooters, the developers of Space Marine essentially said: "Cover is for pussies." There is no crouch button. Health is regenerated not by hiding behind walls and crossing your fingers, but by spilling the blood of His Majesty the Emperor of Mankind's foes.
Which brings us to: Health packs have never looked greener and savage: a mob of Grechin is the equivalent of "here's a bunch of health. Enjoy" (you can use an execution on Grechin without having to stun them, making it "press E for free health"). Fridge Brilliance will have you realizing the "about time, lets wade into them" mentality is how Space Marines are supposed to act.
Another meta example, this one for the character Titus: 1d4chan has a page about him. On it, he is given the highest praise, in spite of being an Ultramarine, a chapter that is widely considered by that site to be a bunch of Mary Sues. They even list the reasons why they think that he's so awesome, including perhaps the greatest single compliment that 1d4chan can give: He Gets Shit Done.
The game opens with Titus (with Jump Pack) singlehandedly dropping down onto an Ork frigate (after avoiding all those shipwrecks in the air), massacring half the crew, and shooting it down before riding the flaming wreckage down to the planet's surface. This is the game's tutorial level.
You forgot the best part. How he managed to drop the frigate? He turned its main gun (firing all the time) against its bridge. Literally.
Simply saying he "turned" it doesn't quite get the image across. He actually grabbed the front of the turret by hand and forced it around by sheer brute strength until it was pointing at the ship's own bridge. Space Marines clearly believe in the direct approach.
This also reinforces why Orks love fighting against Space Marines. Because he pretty much destroyed the frigate in a way an Ork Nob would do too, just with less mad laughter.
The first time you used the ground pound with the Jump Pack. Bonus points if you actually directly hit an enemy on your way down. Double bonus points if you got the Death From Above achievement in just the first jump pack area.
The War Boss should also get an honorary mention. As Nemeroth is summoned, he sics several Bloodletters to jump at the Boss and topples him off of the rather high platform. Later into the cutscene, the War Boss climbs right back up ready to have several goes at Nemeroth, no worse for the wear. As he knocks both of them off the platform, it's clear the ork is having the time of his life.
"I ain't so easy ta kill." With a single whispered line Grimskull establishes himself as the most badass Ork Warboss since Gorgutz.
Beating the hell right out of a Daemon Prince while free falling from a ruined space elevator? Holy shit!
Not to mention literally crushing said Daemon Prince's head with your bare hands.
Marching across a Chaos-held bridge with a Tactical Squad of Blood Ravens behind you and halfway across, a squad of Ultramarines drop in from orbit behind the force you're battling!
This is made especially awesome by the fact that the reinforcements actually have a palpable effect. It's not simply putting good guys in the field so you don't feel alone in the middle of battle, they genuinely start making the crossing MUCH easier. The whole game has gotten its challenge from how Titus' Ultramarine squad has been completely outnumbered, but getting some help from fellow battle-brothers is enough to start turning the tide in Titus' favor.
Titus knows about the Aurelian Crusade and doesn't question the Blood Ravens' loyalty in spite of their previous chapter master and much of the upper ranks being heretical. This suggests that not only is the Space Marine ending of Retribution canon (supported by the Blood Ravens using Angelos' Badass Creed), but that Titus knows of their feats and believes in them enough to have no doubts about warriors he has only just met whose chapter was known for being tainted. Truly awesome for the Blood Ravens.
It might not be something that happens every playthrough, but in this video (https://www.youtube.com/watch?v=qAkZOvlXNu8) when the Chaos Marines see Titus and the Blood Ravens charging their line, even though they have you very much outnumbered and outgunned, the very first thing you hear is one of them screaming "RETREAT!!"
At the very end of the single player campaign, Titus delivers a truly epic verbal smackdown to Leandros who called the Inquisition down on Titus and labeled him a heretic because Leandros followed the Codex to the letter.
Titus: The Codex Astartes is a set of rules. They guide us... shape us as Ultramarines ... teach us to hold duty and honor sacred above all. But how we live with those rules is the true test of a Space Marine. And you... have failed.