Warning! Here there be spoilers for season 5 of Supernatural.
Deal with the Devil; You know how it works. Want to be a millionaire, or to get back at that obnoxious boss? Well travel down to those crossroads and Mr. S will guarantee your wildest dreams, if you just sign on the dotted line with your own blood.
2.22: All Hell Breaks Loose Part 2
Crossroad Demon: Following in Daddy’s footsteps, you wanna make a deal. Little Sammy back from the dead And, let me guess, you’re offering up your own soul.
Dean: There are a hundred other demons who’d love to get their hands on it. And it’s all yours. All you gotta do is bring Sam back. You give me ten years. Ten years and then you come for me.
Sammy’s dead and Dean can’t deal. So, he does what any good Winchester would do. Makes a deal with the Devil. Of course. Why not. He’s pretty much a love martyr anyway.
Death is Cheap; Important characters will have a terrible tendency to die dramatically, but will not, under any circumstances, stay dead.
4.01: Lazarus Rising
Bobby: Dean, your chest was ribbons, your insides were slop. And you’d been buried for four months. Even if you could slip out of Hell and inside your meatsuit.
Dean: I know. I should look like a Thriller video reject.
Bobby: What do you remember?
Dean: Not much. I remember I was a hellhound’s chew toy. Then lights out. Then I come to six feet under. That was it.
Neither Winchester brother is ever going bite it for good. Ever. Death is pretty much an every day occurrence. Like the sun coming up. The eldest Winchester brother excels at dying, horrifically mostly, but he never stays dead. That would suck all the fun out of life and neither Heaven nor Hell is ever going to let him rest in peace.
Future Me Scares Me; So you're a travelin' in time, as you do, when suddenly! Ooh, it's future you! Only... you're different. And not in a good way.
5.04: The End
Dean: Oh, come on, you don't trust yourself?
Future Dean: No. Absolutely not.
Zapped to the future by that bastard Zachariah, Dean comes face to face with himself. And boy is Future!Dean an asshat. Disturbed by Future!Dean’s willingness to sacrifice his friends, and the fact that he is back to his torturing ways, Dean sees his future self as man he doesn’t want to become.
Crowning Moment of Awesome; The moment when a fictional character does something for which they will be remembered forever, winning for them the eternal loyalty of fans.
4.01: Lazarus Rising
Dean: Who are you?
Castiel: I’m the one who gripped you tight and raised you from Perdition.
Badass much? Really, Castiel needs no introduction after that. He is awesome from the very beginning.
Straight Man; Somebody has to set up the joke so the funny guy can deliver the punch line. That's the Straight Man. He rarely gets the funny lines, but has to have impeccable timing and delivery so that the comic (the other half of a comedy duo) can hit it out of the park.
5.02: Good God, Y’all
Castiel: I’m going to find God.
Castiel: Yes. He isn’t in Heaven. He has to be somewhere.
Dean: Try New Mexico. I hear he’s on a tortilla.
Castiel: No, he’s not on any flatbread.
Castiel, the angel with no sense of humor. Who would have thought. His deadpan expressions and perfect timing makes Cas a riot to watch. Especially in season 4. But this exchange between Cas and Dean is one of my favorite moments.
Broken Angel; This is a unique, beautiful creature that has been cruelly torn asunder. Not even killed, but left alive to suffer being flawed after having been "perfect" or at least whole.
5.16: Dark Side of the Moon
Castiel (to God): “You son of a bitch! I believed in…”
Where to start? This particular trope took place over the course of two seasons. It wasn’t until Dark Side of the Moon that he truly went over the edge after he learned of God’s indifference regarding the Apocalypse. And the next episode, where he drank and entire liquor store? And beat the everliving hell out of Dean? Poor, poor Cas. He was beyond broken until the last episode of season 5.