One of the things that in my opinion really adds to the "wow-factor" of a dashboard, is to plot your graphs on a background picture. With Tableau 8, floating dashboard objects is a great improvement. But what would be really great, is to be able to make the chart areas (the white background) transparent.
Let's say you want to create a dashboard for an airline company, which contains three graphs and you want to plot the graphs on a picture of an airplane. You would have to add a fourth worksheet which contains the picture. You then create your dashboard, starting with the picture, which you make full screen. Then you add the charts and set the transparency to the desired setting (0-100%), so the background picture shows through.
I've attached a picture of an existing dashboard (done in Excelsius) to show what I mean.
--> Community Manager update [Ciara Brennan]
Transparent worksheet backgrounds - feature released in version 2018.3