I recently did an upgrade from Windows Vista to Windows 7, and I encountered a few difficulties with the install process, which I documented here.
Windows Mail is Missing
In Windows 7, they removed Windows Mail. Although Windows 7 can still run the Windows Mail program, the upgrade process will uninstall Windows Mail, which is somewhat annoying (I'm not sure where the Windows dev team started having the great idea of uninstalling functionality that was working fine before the upgrade, but I think the new Windows team has some weird priorities that previous Windows teams didn't seem to have).
During the upgrade process, Windows 7 will tell you that it will keep your old Windows Mail settings around, and it will be easy to install Windows Live Mail Desktop later. This is untrue. Windows Live Mail Desktop will automatically import your old Windows Mail settings only if it detects that you have Windows Mail. Of course, Windows 7 uninstalls Windows Mail, so Windows Live Mail Desktop will not automatically import your old settings. So if you use Windows Mail in Vista, you should install Windows Live Mail Desktop FIRST, have it pick up all your old settings, and then upgrade to Windows 7 SECOND.
If you didn't do this, then you have to manually re-enter your account settings, then import your old mail from C:\Users\[userid]\AppData\Local\Microsoft\Windows Mail, and then go into contacts to import your old contact list from Windows Contacts.
East Asian Handwriting Recognition
The whole reason I upgraded to Windows 7 was because the Chinese Handwriting Recognition was a little flaky. It didn't integrate properly with Internet Explorer and Google (it kept losing focus and changing its language), and it was hard to resize the input panel, etc (it, oddly, seemed to work fine in Firefox though). I was hoping that they finally fixed this problem in Windows 7. And they did. They removed East-Asian Handwriting Recognition support from Windows 7. Instead, they want me to shell out an extra $150 for Ultimate in order to get access to this feature, which they removed during my upgrade to Windows 7. To the product manager who earned a nice bonus for thinking of this way to increase Microsoft's revenue: "Thanks a lot, asshole." I remember a time when Microsoft was mean and dangerous, but at least they would try to do right for their customers. Nowadays, their main areas of innovation seem to be finding fiendish ways to increase revenue.
Fortunately, Windows 7 still includes limited East Asian handwriting recognition in the IMEPad (to be removed in Windows 8, no doubt). And it seems to work more reliably than in Windows XP or Windows Vista. The instructions for enabling the Chinese handwriting recognition in IMEPad are the same as for Vista.
No Going Back
I was so annoyed with losing all this functionality during the upgrade that I was going to revert my upgrade back to Windows Vista. Windows has a long history of allowing you to undo even really invasive Windows upgrades (e.g. you can uninstall Windows XP and revert back to Windows 95 after such an upgrade). But, apparently, they stopped providing this functionality ever since Sinofsky took over the Windows Vista team (it might also be related to Windows Activation). So I'm stuck with it. I'm starting to think this will be the last Windows upgrade that I will ever do though, so I'm not too annoyed.