Contractual Genre Blindness: Steve understands that his hilariously frustrated reactions are what the slices come to see. As such, he deliberately avoids walkthroughs, spoilers, basic play tips, and even standard game descriptions in order to make himself as confused/surprised/frustrated as possible. Basically, he suffers on purpose for the sake of your entertainment. You Bastard.
Gosh Dang It to Heck!: Steve absolutely refuses to use most swear words in his LPs and bleeps himself out whenever he yells one accidentally. In addition, he tends to be hilariously scandalized by characters swearing in-game, though he doesn't bleep these.
Steve: Stop saying the s-word! You have a vocabulary, Leon; use it!
Although he does yell things like "Jesus Christ!" and "For Christ's sake!" and clearly has no problem with "dammit" or "hell."
Insistent Terminology: Steve both deliberately and accidentally mispronounces things. Most of these mispronunciations then stick for the remainder of the LP. Of particular note is "montmers," which is used throughout multiple LPs.
"Endo Gladry" (based on a Mondegreen on Steve's part) for the enemy in SCP 087-b.
In his Minecraft videos, he calls the zombies "Zombars", skeletons "Skelingtons" or "Skelly" and creepers "cactuses". He throws a Lampshade on it in the first video of his Hard difficulty playthrough.
Nightmare Fuel: Pretty much anything to do with the Mask of the Child from his Dark Souls let's play. Especially during the beginning of episode 25 (I SEE YOU) and the ending of episode 27. Poor Quinton...
OOC Is Serious Business: Steve adamantly insists that nobody spoil things, no matter how far off he is. If he gets frustrated enough to ask for advice, you know things've gotten really desperate.
Likewise, Steve tends to get very loud and vocal when he's scared and/or irritated (it's what the slices come to see, after all). If he goes silent, you know he's either on the verge of a heart attack or a Rage Quit.
Precision F-Strike: When Steve is sufficiently scared, he'll forget about Gosh Dang It to Heck! and just blurt out the first (and often worst) swear word that comes to mind. Even though he bleeps it, it always has this effect.
Quicksand Box: Steve often becomes paralyzed by large levels and tends to get very, very lost.
Of particular note is his LP of Dark Corners of the Earth, which is played over the course of several years. He goes from relatively enjoying the game to screaming while firing round after round of ammo at lamps to (infamously, thanks to Humon) singing "Cuban Pete" during the boss fight with Mother Hydra.