Folks we are have been there when working with Project 2010. There comes a time when you need to save as your project as 2007 for other users/peers to view your project. Or you receive a Project 2007 file and save it as Project 2010. There are some task features in Project 2010 that are changed when saved in Project 2007 or saving a Project 2007 file to Project 2010.
The two that I have come across many times is:
When saving a Project 2007 file to Project 2010 file --- Cross project links can not be expanded.
So when you have cross-project links in multiple files and some of the files (i.e. subprojects) have not been converted up to Project 2010 – the links (i.e. Project Summary tasks) cannot be expanded. However those links that have been converted up to Project 2010 can be expanded.
When saving a Project 2010 file to Project 2007 file --- Inactive tasks are deleted.
For a complete list, read this MSFT article: http://office.microsoft.com/en-us/project-help/use-project-2010-with-previous-versions-HA010351708.aspx