The calculated metric functionally in Discover is really good but it's missing 3 major things; which are the ability to:
Use rolling dates in a calculated metric
For example: I really want to be able create reports that show week on week change.
Use dimensions in a calculated metric
If I've created a report using dimensions, I have go through converting all the dimensions to segments before I can create calculated metrics from the columns or start from scratch with my calculated metric (both of which are not very efficient)
Use Discover calculated metrics in SiteCatalyst & Report Builder
Discover segments can be used in SiteCatalyst and report builder. It would be great if this principle could be extended to Discover calcuated metrics as it would add more flexibility into the products.