For me, Office is the killer. You cannot seamlessly transfer things such as PPTX files from Office 2007 to OpenOffice. This is a critical problem, and is the thing keeping me on Windows. All of the scientists at my institution in my field use Microsoft software for presentations and for some amount of data exchange (the 'high-level', post-processing stuff--the actual data lives in flatfiles or databases). This, unfortunately, keeps me a bit locked in.
That used to be the case for me, but around here the Office ecosystem has been increasingly fragmenting between versions--- some departments are all on Windows 7 with Office 2007, but quite a lot of machines are still on XP with Office 2003, and some folks (esp. on laptops) are using the Mac version of Office. The interoperability mess that causes is pushing people away from sending out Office file formats at all (mostly moving towards PDF), and to the extent they do, they choose save-as for some old version of Office rather than sending docx/pptx.
I've taken Word 97 files and tried to load them into Office 2007 only to have the document formatting be completely messed up. I mean, I've come to expect that when loading a Word file into OpenOffice, but to have it fail from one version of Word to another is pathetic.
One thing I've noticed is that corporate adoption of wikis is slowly chipping away at MS Word's marketshare. There aren't many times when you genuinely need a word processor today.
We still use MS Excel quite a bit - it's a good program.
I suppose I could just run Wine...