Tables with merged cells, footnotes or just an unusual text colour can all cause problems and Google Docs won't warn you on importing that you're not seeing the document as the author intended or on exporting that the recipient won't see exactly what you wrote. There are also severe limitations on the size of documents you can import. Word-processing documents can only be up to 500KB, presentations up to 10MB (2MB when uploaded from the web) and spreadsheets up to 1MB.
Collaboration
Up to ten people can all edit the same document at the same time (50 for spreadsheets) and any document can be viewed by up to 200 people at once. Spreadsheets and presentations get a "Discuss" or "Chat" pane to enable the collaborators to have an instant message conversation about their changes. It is easy to share documents with other people, you just have to select the document, click the "Share" button and give their email address. They are sent an invitation with a link to the document.
They just have to click the link and, providing they have a Google Docs Account, they can view or edit the document too, depending on the permission you gave them. If they don't have a Google Account they can only view the document. They would need to sign up to edit it. Of course, even two people editing a document at the same time can tread on each other's toes. There is a limit to the usefulness of this kind of free collaboration and sometimes you might benefit from something more sophisticated.
Mail and Calendar
Google Mail and Calendar are both distinctly separate offerings, not really integrating into the other Google Apps except that they will all look at the same list of Contacts for sending email, scheduling meetings or sharing documents. Calendar is starkly functional but does let you set up multiple calendars or share them with friends and colleagues.
Mail brands itself as "different" because it gathers all your emails and the replies to them into "conversations" but the user interface is a confusing mishmash of icons, buttons and hyperlinks surrounded by gadgets which implore you to use them to chat to people or invite them to use Google Mail too. Thank goodness that as a paying customer you can turn off the advertising if you want to.
Offline
One of the major drawbacks to Google Docs has to be that you need an internet connection, and a reasonably fast one at that, in order to do anything. If you're not net connected, you can't do anything. Well, if you install Google Gears on your PC, you can still run Google Docs, just, but there are severe restrictions including the inability to create any new documents.
