Ted: I gotta do what that guy couldn't. I gotta take the leap. Okay, not a perfect metaphor, 'cause for me it's falling in love and getting married and for him it's... death. Barney: Actually, that is a perfect metaphor. (to Marshall and Lily) By the way, have I congratulated you two yet?
Artistic License – Law: Barney mentions The Jail Bait Wait when he says the only reason to wait a month to have sex is if she's 17 and eleven months, implying the age of consent is 18. New York's age of consent is 17. It is, however, 18 in California.
Also, Marshall presumably concentrated on environmental law while he was in law school, since that's what he wants to do with his law degree. He spends more of the series than not working in corporate law. And also gives his friends legal counsel when they need it. You can't just switch back and forth that way- a corporate lawyer would be studying completely different areas of the law than an environmental lawyer would. On the other hand, Marshall winds up as a judge, so he clearly knows his stuff.
A Storm Is Coming: The opening of the season finale foreshadows the massive rainstorm that occurs when Robin and Ted get together while Marshall and Lily break up with both the narrator and a tv weatherman invoking this trope (in the past and future-tenses respectively).
Big Damn Heroes: Marshall in the New Year's Eve episode. It's played that way, with Marshall running through the steam to Bon Jovi.
Bittersweet Ending: Ted comes back to his apartment, high on the thrill of having finally gotten with Robin and more happy about life than he's been in a long time...to find Marshall on the stairs outside, holding Lily's disowned engagement ring.
Bookends: The series begins with Marshall happily showing Ted the engagement ring he will give to Lily. The Season Finale closes with Marshall tearfully showing Ted the same ring disowned by Lily. Further, both the pilot and season finale involves and shows Marshall and Lily having sex after the proposal and before the break-up, as well as Ted riding a cab to go (pilot) and to leave (s1 finale) Robin's apartment.
Bottle Episode: "The Limo" takes place almost entirely in... well... the limo.
Break the Cutie: Kind of happens in "Game Night". Hippie Barney is more funny than sad, and his ridiculous crying similarly distracts from the tragedy, but the watcher is also shocked at how Shannon screwed him up.
Butt Monkey: Natalie. She and Ted dated, and Ted broke up with her over her answering machine on her birthday, during a surprise party at which she hadn't arrived yet (to be fair, he didn't know about that), waited three years for her to get over him, tracked her down, apologized and begged her to take him back, and then breaks up with her three weeks later, again on her birthday.
Comically Missing the Point: Ted's kids, after he tells them the story of when he dated an ex, who he had previously dumped on her birthday, via an answering machine message, convinced her that he'd changed in order to get a second chance, only to break up with her on her birthday AGAIN, only this time in person... Which led to her kicking Ted's ass. All Ted's kids took from this story is that their dad got beat up by a girl.
Complexity Addiction: In 'Best Prom Ever', Barney attempts to sneak into a prom by first crawling through the vents; then borrowing a giant turtle mascot costume to disguise himself... whereas Ted and Marshall snuck in through the back door.
Confusing Multiple Negatives: Ted after getting plastered in 'The Pineapple Incident'- "And now, I don't think I won't not go to the bathroom!"
Lily and Marshall are a cute couple and very much in love. They like wearing matching Halloween costumes. In "Slutty Pumpkin", they dress up as a not-gay pirate and his parrot.
In a flashback in "Slutty Pumpkin", Marshall and Lily are shown being Sony and Cher (surprisingly, Marshall as Cher and Lily as Sonny) for one Halloween.
"Slutty Pumpkin": Barney wants his best friend Ted to coordinate their Halloween costumes. He brought two ace pilot costumes and wants Ted to go as his literal and metaphorical wing-man. Ted defies the trope. He insists on wearing his old costume as the infamous ballot vote "Hanging Chad" because that's how possible future Mrs Mosby known only as Slutty Pumpkin remembers him.
"Slutty Pumpkin": Robin and her boyfriend joked over emails about dressing up as Hansel and Gretel. Mike thought she was serious, but Robin's part of the costume was missing.
Flynning: An episode has Marshall and Ted in a swordfight over Lily moving in/who gets to keep the apartment in the end. Any sense of the fight being real disappears once they start pausing to issue instructions on how next to Flynn in a stylish and awesome manner.
Foregone Conclusion: "Kids, I'm gonna tell you an incredible story - the story of how I met your mother." - these are the first words of the series.
"Because this, kids, is the true story of how I met your Aunt Robin!" - we learn Robin's not the Mother at the end of the pilot!
Also, we know for a fact that Ted, Marshall, Lily, and Robin (but not necessarily Barney - though he is an Honorary Uncle, Ted speaks about him as though his kids don't know him that well) will still be close friends after the series is over, no matter what an episode tries to throw at them.
Fruit of The Loon: The inxeplicable pineapple that shows up after a wild night of boozing.
Funny Background Event: In Best Prom Ever, during a phone conversation between Robin and Marshall, Barney sneaks into the prom, gets caught, and is thrown out.
Good Angel, Bad Angel: Parodied; they're at a rooftop party, and Ted has to pee, but doesn't want to leave. Barney, dressed as a devil, suggests he pee off the roof from behind his right shoulder. A random dude dressed like an angel says from Ted's left shoulder that it'd be a bad idea. And then asks for some weed.
"Good Luck" Gesture: In "Slutty Pumpkin", Lily crosses her fingers intensely when she hopes that she and Marshall will win the costume competition.
Gray Rain of Depression: In the season finale, when Ted finds Marshall crushed after Lily left him to go to San Francisco in order to pursue her dreams.
Grilling the Newbie: The gang does this to poor Victoria about her love life, with the questions thinly disguised as being part of a Truth or Dare style board game.
Hoist by His Own Petard: In "The Duel", Barney beings to institute a "Lemon Law" on all his dates, meaning that he's giving them five minutes before he decides if it will work out or not. Near the end, a woman he tries to pick up "Lemon Laws" him. Rather than being upset, Barney is happy that his idea has caught on, although he regrets not calling it "Barney's Law".
Holiday Volunteering: The first Thanksgiving episode features Ted and Robin trying to volunteer at a soup kitchen on Thanksgiving. However, thanks to this trope, there's no openings. Much to their surprise, Barney is a rockstar volunteer and vouches for them. However, they soon learn that the volunteers aren't exactly the saints they envisioned, and wound up being kicked out.
Hypocritical Humor: Barney calls Marshall "gay" for hosting a wine-tasting while wearing a silver, reflective shirt.
Leaning on the Fourth Wall: In one episode, Barney is telling Ted that the Universe doesn't care about Ted's love life. Marshall interjects jokingly, "Unless Ted's love life is the glue binding the entire Universe together!" Everyone laughs, of course.
The Pineapple Incident - where the hell did he get that pineapple?
Not so Fast, Bucko!: An obvious use: At the start of the pilot, Future Ted starts to tell the story of how he met his kids' mother. However, by end of the pilot: "And that, kids, is the story of how I met your Aunt Robin." Justified, since Future Ted said it would be a long story.
Oblivious to Love: Ted is completely clueless to Robin's growing feelings for him in the middle of the season, that is until Lily tells him.
Plot Leveling: The first 13 episodes, written before the network ordered more, were conceived as if Victoria ("Buttercup") was the titular mother. Once the series was extended, they rewrote the concluding voice over, and Victoria was downgraded to a recurring girlfriend in the second half of the season.
Riddle for the Ages: "The Pineapple Incident". Ted and co, as well as the viewers, will never find out how he got it and why he brought it with himself home from his epically drunk night.
Romantic Rain: Ted has been in love with Robin for the whole season 1. In the finale, he's convinced he has to make it rain to keep Robin in New York (otherwise she will go on a company trip and might get together with her colleague). Amazingly, it starts raining heavily and Ted heads to Robin's. Ted is soaking wet and convinces Robin to go outside as well, though he comes to her door first and they start kissing violently.
In "Slutty Pumpkin", Marshal and Lily are dressed in matching Halloween outfits and share dinner — Lily spoonfeeds Marshal. They later also share a dessert.
Robin is reluctant to do any lovey-dovey stuff with Mike in "Slutty Pumpkin". Mike wants her to taste his chicken, but she refuses and defies the trope. Lily tells her she should be more girlfriend-like and she attempts to share her sundae, but she likes it too much, so it doesn't really work out well.
Seven Minute Lull: In a mild variation, the club music stops right after Ted realizes the girl he's with can't hear him, and starts shouting weird things.
Ted: "I'm wetting my pants!"
Shaggy Dog Story: The pilot. The whole episode centers on Ted meeting, falling for and ultimately failing to have a relationship with Robin, with a promise of many more stories about his attempts, successful or not, to win her. We then learn that she is not the Mother. The entire episode, as well as much of the next two seasons, becomes this trope.
Shock Party: Ted breaks up with one of his girlfriends over her answering machine, while all her friends and family are listening, waiting with a Surprise Party.
Shout-Out: Barney's transformation in "Game Night" bears some resemblance to another famous suiting-up. Especially considering that the episode first aired a few months after that movie came out.
Stalking Is Love: Though Ted surprising Robin in her flat with an entire orchestra isn't received with eruptive joy.
Ugly Guy, Hot Wife: Stewart and Claudia but played a bit differently; she's a controlling shrew and he's a miserable drunk. However, characters repeatedly comment on Claudia being out of Stu's league.
Un Duet: Marshall and Lily have a full dance-routine to go with "Don't Go Breakin' My Heart." Marshall, eager to fit in with his new co-workers, doesn't sing the song at karaoke. However, when he decides Lily's more important, he gets on stage and sings his half of the song, until Lily joins in halfway through.
Unusual Euphemism: When Ted says "It's ineffable", Natalie misunderstands and says "I'm not F-able?!"
Vertigo Effect: Used in "Betty Full of Turkey", when Lily finds out the typical size of an Eriksen newborn.
Walk On The Wild Side Episode: "The Pineapple Incident" has the gang convince Ted to drink instead of think. He wakes up next morning with a sprained ankle, burned coat, several drunken messages on Robin's phone and a girl he's never met before in bed next to him. Oh, and a pineapple that never gets explained.
What Happened to the Mouse?: Robin had a group of friends in the pilot and another one in the episode where she hangs out with Barney. They all appear in one scene, never to be seen ever again. Robin even talks about how she barely knows anyone in New York and has no friends before meeting Ted, since she's new in town and she spends most of her time at work.
Will They or Won't They?: Ted and Robin for the majority of the first season; subverted and further complicated by the fact that the audience knows from the first episode that the two do not end up together (so season 2 is will they or won't they break up?) — and even more so because they're suddenly willing again.