I have a farm running MOSS. I have a user running IE9. When he tries to open a document from a library he's prompted to download it, all the office integration features don't work, but work fine for everyone else. It's as if he's using a non-IE browser.
What client-side factors can cause office integration to break?
Make sure you are running the 32-bit version of IE9. There are quite a few limitations if you don't have the 32-bit Activex controls (see this TechNet article). You should also check that you have the 32-bit version of Office installed (I don't think the 64-bit version has the controls).
Check the browser configuration against the settings on computers where it is working, particularly ActiveX scripting, etc.
Some client features don't work if you are running on a server OS (solution: enable the "Desktop Experience" Windows Server feature).
One cause can be, that the "WebClient" service is stoped. I know that is plays a role for Explorer view /Datasheet view. Maybe it is worth looking at it, even though I am not sure if it is impacting the office integration features.
It sometimes happend in our company to, on computers with windows 7, (IE 9) and 64-bit Office inside. "Repair" installation of Office apps. can help you, it is almost certainly problem on client side think. There is some driver or connection that stops working, it is quite new issue for us (2 - 3 weeks), so we don't have time for investigation yet, but Office reinstall is fast solution for now.
