In MOSS if you published an Excel workbook to be displayed in Excel Services that contained "Unsupported Features" you would most likely be greeted with an error when the web part would try to render any part of the workbook (e.g., worksheet, named region, object).

This behavior is described in this MSDN article.

You'll notice the MSDN article describes that certain features "... are not displayed" as well. This behavior is expanded in the SharePoint 2010 version of Excel Services so that no features in a valid Excel 2007/2010 workbook will prevent the web part from rendering the content that IS supported.

Excel features that are unsupported is SharePoint 2010 Excel Services will now simply not be displayed.

While this may cause confusion occasionally (dude, where's my data?), the overall end-user experience will be much better; I can attest to troubleshooting a workbook that wouldn't render only to find that a comment had been attached to a field!