Both the Excel Web App and Google Docs' spreadsheet app support the standard spreadsheet functions, including creating charts. Both also have the capability to create Web surveys - called forms in Google Docs.
The Excel Web App supports more Excel functions than Google Docs does, so complex spreadsheets may transfer over more easily. However, it does not support macros, does not allow you to freeze header rows, and won't let you email a copy of the spreadsheet as an attachment right from the application.
The Google version supports scripts, freezes rows, and lets users email a copy of the spreadsheet from within the application in Excel, PDF, or CSV format.
The inability to freeze rows and better collaboration gives Google Docs the edge for Web-only users, but better integration with Excel gives the Microsoft app the edge for existing Office users.
Presentations: Advantage Microsoft
The PowerPoint Web App offers a choice of nine starting templates, while Google's presentation app offers 20 - but the PowerPoint templates are nicer than Google's.
Both have all the basic editing tools, and ability to share the presentations with the public and embed them into websites.
Note taking: Advantage Microsoft
Microsoft has the OneNote Web App, which, like the other Office Web Apps, is a less-functional version of the original. Most critically, it is still missing a print functionality.
However, it's a big improvement over Google Docs' Notebook app, which was completely shut down about a year ago.
Graphics: Advantage Google
Google Docs has a nice application for collaboratively creating simple graphics and charts online.
There is no equivalent tool in the Office Web Apps.
Mobile: Mixed bag
The Google Drive mobile allows editing of word processing documents right from the app, and viewing of spreadsheets, presentations, and graphics. There's an option to open the files in other applications as well.
The SkyDrive mobile app shows previews of word processing documents, spreadsheets, PDFs, and presentations, but not OneNote notebooks. As with the Google Drive app, there's an option to open the files in other applications.
Both SkyDrive and Google Drive can also be accessed from a mobile browser like Safari, and both default to a mobile-friendly interface. Google Docs, however, allows mobile-friendly editing of spreadsheets and word processing documents. Google also easily switches into "desktop" mode, which allows full editing of graphics files.