RGoogleDocs doesn't work with the new Google Sheets
As far as I'm aware, there's nothing that can be done about this at this time, but I thought it would be worth pointing out anyway.
According to the page at https://support.google.com/drive/answer/3543688?hl=en&ref_topic=20322# the new version of Google Sheets does not yet support API calls. Any spreadsheets created under the new system will not yet communicate successfully with any API calls, including RGoogleDocs. This is irrespective of whether the user making the call has upgraded.* When the API feature is enabled for the new-style sheets, it is possible that this library might need tweaking to take account of it; that will all depend on whether there is a change to the syntax of the API calls needed, which is up to Google.
This comment was written on 2014-02-07; if I notice any change to the situation I will try to update with another comment, but I can't guarantee it, so please check the Google site for the latest news. They also have workarounds and a description of how to downgrade to the old documents. Downgrading does not change spreadsheets created under the new system.
*The call will fail in this scenario: a user who has upgraded to the new Google Sheets creates a sheet and shares it with another user who has not. If either user tries to access the sheet through the API, the call will fail. In other words, the call will succeed or fail on a spreadsheet-by-spreadsheet basis, not a user-by-user basis.
In R, the error message I get is "Error in getFeed(doc) : trying to get slot "worksheetsfeed" from an object of a basic class ("NULL") with no slots" If you experience a similar message, I recommend looking on Google Drive and checking if the spreadsheet is under the new system whether or not you have upgraded to the new sheets on your account.