The appeal of Castiel has always been his “fish out of water” story, especially his inability to grasp even the most common pop culture references. Most of Dean's jokes are met by Castiel with blank stares and total incomprehension. Then there's things like Castiel sitting on a whoopie cushion. After a long time of letting the noises happen, Cas only has one thing to say.
(Blankly) That wasn't me.
There's Castiel's very serious search for God in season five's "Good God, Y'All."
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.
Then there's the encounter with cupid in "My Bloody Valentine":
Castiel: What human myth has mistaken for 'Cupid' is actually a lower order of angel. Technically it's a cherub, third-class.
Castiel: Yeah, they're all over the world. There are dozens of them.
Dean: You mean the little flying fat kid in diapers? Castiel: They're not incontinent.
Or this conversation with Dean in "The Third Man":
Dean: No, not later. Now. Stop, all right? Too many angels, Cas! I don't know who's on first, what's on second.
Castiel: What is 'second'?!
Dean: Don't start that.
Or there's this from season eight's "Hunteri Heroici":
Dean: She's right, you know. I mean, the whole heart jumping out of the guy's chest, the – the – the delayed fall – that's straight-up Bugs Bunny.
Castiel: So we're looking for some sort of insect-rabbit hybrid? How do we kill it?
Yes, the missed references are numerous and go on for years. That all changed in season nine’s “Meta Fiction” though when Metatron downloaded the meaning of every pop culture reference to Castiel’s brain. Suddenly he was blowing Sam and Dean's minds when he actually got their Star Wars reference. Did Cas suddenly learn from all this? Not really, considering he just earlier this season adopted his own FBI alias of “Agent Beyonce.”