Office Web Apps give users a browser-based viewing and editing experience by providing a representation of an Office document in the browser. When a user clicks on a document stored in a SharePoint document library, the document opens directly in the browser. The document appears in the browser similar to how it appears in the Office client application. The Web app also provides many of the same editing features as an Office client application.
When you try to open a Excel workbook in Excel Web App, you might see an error message that tells you the workbook contains features that are not supported by Excel Web App. Unsupported features are contents or functionality in a workbook that prevents the workbook from being opened for editing or, in some cases, viewing in the browser.
You can click on the "Details" link to view the message details and then the "Learn more about unsupported features" link to view information on the unsupported features.
Most common reasons are use of Comments, Worksheets with protection, Information Rights Management (IRM), OLE objects and, data validation in Excel sheets.
If you get the “Unsupported Features” message. you can use the <<Open>> command to open the document in Excel client application. Office Web Apps uses an add-on in your browser to open documents in your desktop program. If the Open command doesn’t work, then the only option is to download the document to your computer, work on it there, and then upload it back to the SharePoint library.
The main Office Help articles to refer to are:
There are lot of keyboard shortcuts provided for Office Web Apps. In case you are not getting the one you need, refer to following articles:
Office Web Apps SP1 provides the latest updates for Office Web Apps, please refer to the Description of Office Web Apps SP1 and Patching Office Web Apps to SP1 and June CUs including SharePoint Server 2010 for upgrade information in case you were waiting for one of these features. Before upgrading, do not forget to read Known issues when you install Office 2010 SP1 and SharePoint 2010 SP1 :-)