Arc Words: The phrase "Blended Family" grates on the nerves after four seasons.
Ascended Extra: Sheldon Schlepper, who was originally supposed to only be in one episode.
Be Careful What You Wish For: Casey learns this the hard way on her sixteenth birthday when she wishes Derek out of her life. But considering that it's all a dream, and the audience is well aware of it...
Belligerent Sexual Tension: Speculation abounds regarding this between Derek and Casey. Thick enough between Truman and Casey to serve up with an ice cream scoop.
Brilliant but Lazy: George managed to become a successful attorney despite having been a slacker and a ne'er-do-well in high school. Derek arguably qualifies, as well.
Brother Chuck: Noel. His last appearance in an early season four episode had him coming to terms with him being just friends with Casey. It was an obvious set up for a Platonic Life Partners friendship or possibly another Love Interest for Casey. But then Truman came in, and Noel never showed up again.
Canada, Eh?: Generally averted, but not altogether avoidable in a self-consciously Canadian show. At one time, averted and mocked in one episode.
And the accents. Dear Lord, the accents.
Canadians have accents?
Not so much accents as "verbal quirks" (which I'm sure us Americans have from their POV). Pronunciations such as Sorry being said as "Sore-ee", Been as "Bean", Again as "Uh-Gain", and of course the pronunciations of the "ou" sound in words like House, About, etc. And no, it's not the "aboot" pronunciation (no idea where that even came from), they just say it... very differently.
The three younger siblings raise some serious Cain when they're expected pick up the slack around the house when Casey and Derek have to study for finals, without any additional consideration or compensation.
In The Movie, Casey stands up to her grandmother Felicia in defense of George and the rest of the Venturi clan.
The Cast Showoff: Ashley Leggat, who plays Casey, is a professionally-trained dancer, and the show emphasized this repeatedly in later seasons. The TV-movie Vacation with Derek even made a whole subplot out of this, casting So You Think You Can Dance Canada winner Nico Archambault as her love interest.
Leggat is a triple threat entertainer. She is also a singer, having sung a few times on the show.
Catch Phrase: Casey's annoyed reaction of "Der-ek!" The distinctiveness is in how she says it. Lampshaded in the episode "Open Mic Plight" when Derek responded:
Derek: Ca-sey! Sal-ly!
Character Development: By the end of the series, Casey is much less of an obsessive-compulsive Control Freak and has learned to relax and enjoy life, and Derek has gotten over his fear of commitment and does right by his family more often than not. That's not to say there aren't still times when they're positively insufferable.
Drugs Are Bad: Casey attempted to intervene when she thought Derek was selling drugs (and using Edwin as his bag-man.) It turns out he was just selling office supplies his dad stole from work, and CDs.
Eagleland Osmosis: This show provides a fascinating look at the similarities and differences between Canadian and American culture. The show itself could be considered a case of reverse osmosis, due to its popularity in the States.
The Klutz: Casey, who is given the nickname "Klutzilla" very only on. However, in later seasons, she is also shown to be a talented dancer (much like the actress who plays her), so it's hard to buy her pratfalling as genuine.
Locked in a Room: Derek and Casey get locked in the bathroom in "The Party" and bonding ensues. Although... whether or not Derek was being sincere is up for debate...
London England Syndrome: An interesting aversion, in that we eventually learn the show is set in London, Ontario (population 350,000, about halfway between Toronto and Detroit), but they never come out and call it "London, Ontario".
Running Gag: 'DER-ek-!' The more emphasis on the first syllable, the angrier Casey is when she says it. Inverted in 'Futuritis', when she says it (as Derek appears on stage) in a very soft, loving fashion.
Say My Name: DER-EK!!! Casey isn't the only one who says it like that, either. In one episode, they lampshade it by having the characters bet as to whether Derek can get everyone to say his name like that. And yes, he wins. It's also become a very popular and simple drinking game, much like "Hi, Bob!". The rules? Drink whenever anyone says "Derek".
This Is My Side: When Derek and Casey were staying at the house alone while the rest of the family went on a road trip, Casey taped parts of the house pink (her side) and blue (Derek's side) so that they would avoid running into each other.