Google Docs has a new validation option that spreadsheet jockeys are going to appreciate. You can now give cells prerequisites, so that you or other users must fill them in with a certain type of data. It can also be set up to provide instructions that pop up as soon as someone starts entering data--similar to what's available in Google Docs' form tool and in Microsoft Excel. Both options can be set inside of the data validator that shows up once you've selected a cell or range of cells.
The Google Docs blog has suggested this as a great way to make sure people who are collaborating on a spreadsheet don't start inputting data the wrong way. The example given is e-mail addresses, where you can set it to validate an e-mail to make sure it's got the .com and the @ in the correct places, keeping you or someone else from having to go in and re-enter it later.
Another good use is the date checker, which will keep everyone who is working on a spreadsheet from entering different time and date formats. This is immensely helpful if you're working on something with people from different countries who may use various formatting localizations.
There are unfortunately a few shortcomings with the new feature. For instance, you can only apply one validation rule per cell, and the editor does not allow you to add multiple validation elements at the same time. Likewise, if you've entered data that does not meet the cell's validation rules, Docs will simply erase your entry unless whoever set up the validation ticked the option to allow users to enter data that does not meet the requirements--something that's not the default.