2. Continuity Of The Show
One of the best parts about the show is this - no matter how small the detail might be, the producers remember it, and use it for later. In short, the show has continuity and that is excellent. In various shows, some parts are just left out or forgotten, but in How I Met Your Mother, the simplest parts are remembered forever, and the viewers need to catch on to them, since they might be useful later on. The references in the show are from their own episodes, and one needs to be careful and vigilant while watching an episode, since they might miss out on something special, if they blink. Small parts to big segments - everything is important in How I Met Your Mother. You would never know which part might come up later, and thus, for fan who will be watching HIMYM for the first time, it's a request to you to enjoy the product, and watch it carefully, to completely experience the magic that the show brings.