I guess it's not the most important thing in the world, but it made it so that it felt like Overwatch didn't have anything new to offer. And it also meant the casual playerbase left the game for greener pastures. I want to look forward to stuff in OW again. Of course the main game mechanics come first, but having stuff to just try out that is different makes the game fresh.
Also, the Workshop basically proved that there is no excuse for the lack of content here either. In a little over a week, players have come up with dozens of unique modes using third person code. If they just had access to a map editor and were able to create some art assets, we'd be having unique events every month. For christ sake, someone's already made a pretty advanced Moba of Overwatch, with attacking towards and everything. But they weren't capable of a new summer games mode in 3 years time?