Besides the API, besides the hyper-local (and, in my experience, _always_ accurate) forecasts, besides the excellent visualizations, besides the nice web app, what I miss most was the history.
I loved the history. It was perhaps DarkSky's least appreciated feature.
You could search (at least within the US) for any date in the past few decades, and find the temperature and precipitation and whatnot for a given location.
You could see what the weather was like on those important dates in your life. You could see what the weather was like the day after those important dates in your life.
It's a little bit of memory that's been excised from the commons. It still makes me sad.