Talking of gimmicks, the show has its catch-phrases, and better than that so do the characters. To make it more interesting, the characters play on those catch-phrases much as any group of friends would. Again, this brings back the realism as one character ribs another about their catch-phrase. The only other series I can think of that does this is Only Fools and Horses. Not bad company to keep.
Wrap-Up
From an indie film-making perspective, How I Met Your Mother is the modern standard. If you're going to make a sit-com, comedy or drama, then you should watch this and work out for yourself what makes it tick. I used to suggest Only Fools and Horses as a good reference point. As surprised as I am to say this, HIMYM now competes with that. I really didn't expect to be saying that when we started watching the series. In once crucial way, it surpasses Only Fools and Horses, in that it is more current. Better still watch both and learn a lot about what you want your characters and actors to be doing.
Watch it, it'll be legend...
...wait for it...
...dary.