The current beta notice for VisualEditor is currently easy to miss. While it uses a circled "?" and the text "BETA", it can blend in with the rest of the VisualEditor editing interface pretty easily (I only really noticed that there was any kind of warning today).
The notice that VisualEditor is beta software should be made more prominent.
James has suggested showing a more prominent notice for first-time VisualEditor users (presumably based on a cookie).
My preference is to make the notice dismissable (using a hidden user preference).