Modern Family is possibly the most popular traditional family sitcom on the air today, and so relies fairly heavily on the established tropes of sitcom plotting. There’s nothing wrong with this, per se, but it can begin to feel a bit stodgy, and when an episode like "New Year’s Eve" comes along and lets things breathe a little, you realize just how annoying the episode that came before it truly were.
The show has a tendency to lean on the plot device of someone having to keep some kind of information or plan secret from one of their loved ones. It’s an easy engine for conflict, but when overused, it can make us forget why, exactly, any of these people are spending time with each other to begin with. So when "New Year’s Eve" throws each of the couples into their own boats, and pushes them off to have fun and bump up against conflict together, it’s more fun than the show has had in a long time.