One of the best features of Google Docs is that you can collaborate, and I don't mean check-in, check-out collaborate like SharePoint. I mean everyone can edit the same file at the same time without having to download or upload anything. Google isn't unique in this feature. Zoho Writer also supports simultaneous editing by multiple users, but you can only have three users before you have to start paying for services, while Google Docs doesn't limit the number of free collaborators you can add to a document.
I've seen some documents completely open to the Web and allowing everyone to edit them. Others are closed and only editable by small groups. You can also set your sharing preferences for a folder and have all items you add to that folder automatically share with a group.
You've got documents, spreadsheets, and presentations. These are online slide show presentations, and now you can even add animated transitions to your slides. (Use this power for good and not for evil.) Like everything else, you can share and collaborate with simultaneous users, so you can work on that presentation with your partner in another state before you offer your presentation at a conference. You can then export your presentation as a PowerPoint or PDF or deliver it directly from the Web. You can also deliver your presentation as a Web meeting. It's not as full featured as using something like Citrix GoToMeeting, but Google Presentations are free.
4. Make FormsI often get oohs and ahhs from the audience when I show people this feature. You can create an easy form from within Google Docs that asks different types of questions and then feeds directly into a spreadsheet. You can publish your form as a link, send it in an email, or embed it on a Web page. It's very powerful, and very easy. Security measures might force you to pay for a product like Survey Monkey, but Google Docs sure does a great job for the price.
I find it useful sometimes to just browse through the templates to see some of the creative ways people use Google Docs.
8. Upload AnythingYou can upload just about any file, even if it's not something recognized by Google Docs. You've got a finite amount of storage space (1 gig) before Google starts charging, but you can upload files from obscure word processors and download them to edit on a desktop computer.
That doesn't mean you should underestimate the types of files you can edit from within Google Docs. Google Docs will convert and allow you to edit Word, Excel, and PowerPoint files. You can also convert and edit files from OpenOffice, plain text, html, pdf, and other formats.
Google Docs even has a built in OCR to scan and convert your scanned documents. This option may take a little longer than regular uploads, but it's worth it.
You can also use an Android app to edit your docs from your phone.