Since most developers and publishers have done away with the "demo" of years past that let us try before buying, I'd say Let's Play videos are the new demo.
I have bought games due to Let's Plays (Splt/Second, Sonic All Stars Racing Transformed), and I have also avoided games due to Let's Plays (Tom Clancy's Division).
Step up, devs. Your game has to be fun to play, as well as watch.
milesprower06's comments