Here's the play by play:
* The way the characters' relationships to each other are so subtly enforced, even in episode's not explicitly about them (my evidence: the fact that every time Robin's made a joke since the third season, Barney has laughed at it and given her a look like she's the perfect woman.) It's not anything the characters need to even comment on, but they function like real people in the smallest of small reactions to each other.
* When the gang makes challenges to each other.
* The fact that the HIMYM crew is basically your friends, only way more fun.
* the fact that when I saw that old woman jumping on the cab, I thought HIMYM looked like the worst kind of sitcom cliche. But I kinda giggled when it actually happened in the episode.
* When Marshall randomly bursts into machines that are both witty, but also character-appropriately stupid.
* The sentimental goofy episode summarizing moments.
and of course...
* Barney. Peddling a bike to see Woody Allen, football tackling Ted, Smelling incredible... Barney.
Happy International Suit Up Day, y'all!