They had a wonderful graph of various weather parameters in a nice slippy zoom interface that made it easy to visualize the forecast. It was built in Flash and they decided that a HTML version wasn't viable due to declining ad revenue.
Some appreciation and screenshots of the old version:
https://www.linkedin.com/pulse/sad-day-weatherspark-dashboar...
https://flowingdata.com/2011/03/14/weatherspark-for-more-gra...
https://www.reddit.com/r/weather/comments/4hkbq4/rest_in_pea...
https://www.glerl.noaa.gov/pubs/fulltext/2016/20160004.pdf - The interface was even noted in a NOAA paper.