What are the differences between Gryffindors and Slytherins?

I think it Pottermore that mentioned that Gryffindor and Slytherin are more alike than first expected. So what makes them similar? What makes them different?

Two main ones seem to be 'being the saviour vs. being the survivor' and the (mostly unfounded) 'good vs. evil'

Gryffindors, what sets you apart from the Slytherin? Are you all Big Damn Heroes (I find it more likely that was mostly just Harry being Harry)? And snakes, what traits do you have (or not have) that stops you from being in Gryffindor?

How does the strength of moral principle play into all this?