Whether it's good or bad (or something in between - some things aren't black and white) depends on how fixated you're in the way things work in older Windows versions.
Many (if not most) complaints about Vista seem to be about UAC and the new security model where the default user isn't a "real" administrator anymore. Too many Windows applications have assumed that the user has administrative rights and can do anything. That changed in Vista, and so those applications either cause a lot of UAC prompts or won't work properly. Of course from user's point of view it's Vista's fault.
Other thing that people still keep whining about is the hardware requirements. Computers that run XP well (which, in itself, is very subjective) might not run Vista so well. Maybe people have short memories, or maybe they're too young to have witnessed, but the same applies to previous versions of Windows as well:
- MS-DOS runs well on a 286 with 2 MB of RAM, but Windows 3.1 might not,
- Windows 3.1 runs well on a 386 with 4 MB of RAM, but Windows 95 might not,
- Windows 95 runs well on a Pentium with 32 MB of RAM, but Windows XP might not,
- Windows XP runs well on a Pentium III with 512 MB of RAM, but Windows Vista might not.
Yes, I skipped some versions and didn't specify any speeds etc., but I hope you get the idea.