Air Vent Passageway: Inversion; Kumar breaks into a police station where Harold is being held to bust him out. And then subverted when Kumar drops through the vent onto the floor. Thankfully he made a decoy call so the station's empty.
As Himself: Neil Patrick Harris, though it's a subversion as he specifically requested to be credited "as Neil Patrick Harris" instead of "as Himself", so that people didn't think he was actually a drug-sniffing, car-stealing womanizer.
He also claimed to have had sex with Whoopie Goldberg's stand-in while filming Clara's Heart at age 14.
Asian and Nerdy: Harold's co-workers cites this as justification for tricking Harold into doing his work.
Zigzagged with Harold. He's an uptight neat-freak who lets bullies from his office and neighborhood walk all over him. But he's also a stoner and an expert at beer-pong. He later ends up standing up to his bullies and screwing them over magnificently.
Subverted by Cindy Kim and her East Asian Club. They appear to be straight-laced nerds, but turn out to be drug-using party animals.
Black Is Bigger In Bed: Harold gets locked up in jail with a philosophical black man, who is resigned to his fate.
Harold: I don't understand how you can be so calm about all this.
Tarik: Look at me. I'm fat, black, can't dance, and I have two gay fathers. People have been messin' with me my whole life. I learned a long time ago that there's no sense gettin' all riled up every time a bunch of idiots give you a hard time. In the end, the universe tends to unfold as it should. Plus, I have a really large penis. That keeps me happy.
Bread, Eggs, Milk, Squick: "If you boys'd like to go on inside, get yourself something to drink, worship, fuck my wife, watch TV... anything you want!"
Butt Monkey: Harold. Exploited at work because of his ethnicity, has parking spaces stolen from him by extreme sports enthusiasts, his best friend uses his scissors for manscaping, he gets attacked by a raccoon, Neil Patrick Harris steals his car, he gets arrested for jaywalking (well, admittedly he also unintentionally punched the cop in the face), his laptop gets destroyed, he gets clotheslined by a tree branch while riding a cheetah, and on top of all of that, he's completely unable to talk to the girl he has a crush on. Harold isn't just the Butt Monkey; he's distinctly headed into woobie territory.
Food Porn: The White Castle commercial that sends Harold and Kumar on their quest. It even includes sleazy-sounding porn music. There's also the Burger Shack drive-thru guy's description of White Castle burgers.
Funny Character, Boring Actor: John Cho and Kal Penn have both said their real-life personalities are more or less the reverse of their characters'.
Gilligan Cut: Immediately after Kumar insists Harold doesn't need to go to the hospital, we cut to him sitting in a hospital waiting room.
Manly Tears: What happens when Harold and Kumar actually made it to White Castle and feasted on those tiny, delicious burgers.
Market-Based Title: Known as Harold and Kumar Get the Munchies outside of the US and Canada, because White Castle operates solely within the US, and it can be presumed that most Canadians are familiar with the franchise as well.
And only in the Northeastern and Midwestern US, at that. This left viewers in the South and the West Coast confused, as they didn't know what White Castle was.
Strangely enough, it was screened as Harold and Kumar Go To White Castle in Singapore, the Philippines, Australia, Belgium and Canada, all of which do not have a single WC restaurant (White Castle's management seems to not want to do international franchises).
Actually, White Castle has a line of frozen foods featuring their signature sliders available in grocers and vending machines since 1987. Many people probably didn't know it was an actual restaurant at the time.
No Fourth Wall: The DVD menu for the first film, where both characters comment on the options available as well as how long the viewer takes to make a selection.
Offscreen Teleportation: When our titular heroes want to cross a street at a crosswalk. Kumar convinces Harold to jaywalk, and Harold checks both directions, to find the road clear on both sides for miles. Then he steps off the sidewalk and a cop car instantly pulls up from just off-screen. Cue Kumar trying to ignore Harold's Death Glare.
P.O.V. Sequel: As noted below, the actors playing the main characters were Funny Foreigner characters in other gross-out comedies and the movie is sort of a perspective shift to focus on them; Harold's boss is a parody of the Jerk AssDesignated Hero of most of these films and amusingly, all of his adventures happen offstage. Similarly, the two have a pair of even nerdier friends Rosenberg and Goldstein who also have adventures off-stage, alluding to the play Rosencrantz and Guildenstern Are Dead which is a famous example of this.
Rant Inducing Slight / "The Reason You Suck" Speech: Both Harold and Kumar. Kumar gets his first when an asshole cop gives Harold a ticket for jaywalking after putting one foot down off the curb; Harold's comes when he runs into his co-workers at the White Castle, catching them lying about an "important business meeting" as an excuse to make Harold do all their work.
Rock Bottom: Immediately after complaining that he doesn't know how much more he can take, Harold gets attacked by a raccoon that snuck into the car.
Ugly Guy, Hot Wife: Freakshow, who is covered in boils (some of which have pus squirting out of them), is married to a rather good-looking woman (played by Malin Akerman (Granted, this is before she gained that bit of weight)). She cites his beautiful singing voice as the reason she fell in love with him.
“Well Done Son” Guy: Kumar isn't interested in being a doctor like his father is. He has an epiphany at the end of the first film and decides to give it a shot anyway.
What the Hell, Hero?: Neil Patrick Harris attempts to avert this at the end of the first movie by approaching Harold and Kumar, apologising for his role in the previous night's events, and offering to pay for their breakfast. They then launch into this trope anyway, leaving him to irritably point out that he's aware of the dick move he pulled, hence the apology-and-offer-to-buy-their-breakfast.
Also $200 for Harold's car. He made some "love stains" in the back.