The biggest difference is there's no extension to install (not a big deal to most). Also, we're focused on non-designers being able to get their thoughts/likes across easy to designers.

That said, as a designer myself the Scratchpad process is largely based on an existing process I've employed for a while to get things going to show other's in a simple manner what i'm thinking and get early buy in to be able to move forward faster and on my own terms.