It would be great if the Tableau interface with R allowed a user to quickly and seamlessly access data in R data frames. This way you could have the power of R with the viz of Tableau, without having to output a file and import it into Tableau. For example, using melt in R is very powerful and would be useful to port over to Tableau quickly.
R is open source, so Tableau should be able to figure out how R data is stored, not only on storage, but in RAM. Could be speedy!