Woah! There are so many features that I am not sure I will need them all. I don't even use half of stumbleupon atm.
However I will keep this thing in mind, in case I feel stumbleupon is lacking something and diigo might have it. I don't want to discourage anybody from using this though, it just seems too much for me.
I already have so many plugins installed:
Diigo - social bookmarking and annotation
If someone from dc.com tried it, I'd be very interested in a supertiny review