Cannabis causes memory loss. No it does not. This is an urban legend. The New Scientist has published several articles debunking this myth. Alcohol use is far worse for memory loss.
Cannabis causes lack of motivation.
Perhaps when stoned some folk would not want to set the world on fire but afterwards many cannabis users find they do their most inspirational work. Just ask in Silicon Valley how many of the programmers come up with their great ideas. The computer revolution has been linked to cannabis use. Ask most musicians since the turn of the century what effect it has had on their music. With out it we would not have had Jazz, the Blues, Swing or Rock and Roll etc.