A hypothetical viewer going into The Bourne Legacy who hadn't seen any of the previous Bournes, and had no idea who Jason Bourne is, would be confused to the point of irritation by this movie.
That's not meant as a criticism, exactly, but an observation. Legacy assumes from the get-go that you're at least kind of familiar with the Bourne franchise, and I suspect one would get more out of it if one had a better memory for the details of the previous films than I do.