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Compare the reasonably full Excel Starter interface with this truly stripped-down Excel Web App. The Insert tab has a mere two commands available, and many formatting and formula option are completely missing. (This screen is from a SharePoint site, but the SkyDrive app looks the same.)
As with Word, the feature set in Excel Starter is identical to the full version where it exists, as in these three tabs worth of commands for creating and editing charts.
One surprising new feature that is unique to Office Starter is the ability to create a portable version of Office that runs from a USB key. The setup program downloads some files and then configures a USB flash drive to run Office (on Windows only, naturally).