The only issue I've noticed upgrading from XP to 7, is the different way in which they handle thumbnail databases. In XP, thumbnail databases are stored locally in the file with the images. In Windows 7 they are stored globally away from where the images are kept. There is both an upside and a downside to this. On the upside, it allows for on the fly resizing of the thumbnails which is quite useful. On the downside, since the database does not go with the images when they are burned to a disk or otherwise moved, the database must be rebuilt each time the disk is inserted in a drive. Also, if the database gets corrupted in XP, you could easily delete it and it would simply rebuild. In 7 this is a bit more difficult to address.
Of course, this is a minor issue considering all the improvements in 7 over XP and really only noticeable when you have a huge number of images to deal with. If your computer is up to the task, then Windows 7 is a great upgrade.