StumbleUpon was "unique" in the aughties sense of the word. Before Reddit and Digg made things Hot or Not this was the place to go for interesting content. It wasn't loud or or overly verbose; it sent you to great sites before SEO and paid media killed content.
Maybe I'm just waxing nostalgic on the implementation as well as the time. IDK.
I hope Garret, Ali and the team at Mix do great things.