Today I upgraded Drupal from 4.6.x to 4.7.x. This came with some special problems, but none of them were in drupal itself, but in modules. To wit, the ecommerce module summoned satan all over itself; it is using the new method in drupal where modules can create their own database tables, and it's not creating them all. I went so far as to disable all the modules, manually remove the tables, and then put the modules back in - which changed nothing. I wasn't actually selling anything anyway, so I went ahead and kept the upgrade.
Freelinking also fails to work properly.
On the plus side, things are now a bit faster, and I installed the jstools module; with the addition of a few other things the site has gained some DHTML style, among other niceties.
4.6.x development has stopped on pretty much all modules so it was absolutely necessary to finally upgrade. Let's hear it for drupal... and let's all pray that 4.8.x has a proper installer module so that we don't even have to unpack tarballs any more.