In the Ask Data interface there is an enriched tooltip which appears on hover over the dimensions and measures on the left of the pane. This shows a preview of the values which are in each dimension or measure. For instance, if you were to hover over a 'Country' dimension you would see a bar chart which shows the most common values; i.e. which countries appear the most in the data set.
It would be great if there were similar functionality available in Tableau Desktop or Wed Editor. This would enable users to preview and get a sense of their data before bringing it into the pane for analysis. Currently the only alternative in Desktop/Wed Editor is the 'Describe' function but this would be much more effective.