To answer the OP's question, when Vista first came out...
Network file copy ws completely broken. It would take forever to copy large files with every system I tried.
My USB peripherals and my Sound Blaster Live PCI card plain did not work. One computer I have with a nForce chipset, a 2Ghz CPU, and 1GB of RAM refused to run Vista.
UAC was overly annoying. Some important (to me) software programs didn't work when it was on. Some important (to me) software features didn't work at all. (QuickBooks QBXML interop for example).
Little things annoyed the hell out of me. I don't like the breadcrumb metaphor extended into explorer the way that it was. I did not lke the breadcrub path bar. I liked to see and edit the full pah always. I liked being able to press backspace to go "up" a folder and was annoyed that backspace now went up. I was annoyed that you can no longer easily change file type icons. The control panel was a mess in my opinion, you know ... moving away from applets with dialogs that had property sheets towards a "web like" explorer view. Not good IMO.
The Windows theme API introduced in XP was depricated for no good reason.
In general, Vista ran slower and hardware drivers were unstable. I believe Microsoft is partly at fault for this for changing the driver model quickly a few times leading up to the release of Vista, pulling the rug out from under hardware vendors causing them to lose confidence and dollars chasing Microsoft's changing dates.
There was also the whole Vista Capable Intel IGP fiasco.
Most importantly, Vista offered people little to no benefits over XP. In most cases it was beneficial to stick with XP, as you could be reasonably sure all hardware and software would work with it, whereas with Vista it was questionable.
I could go on, but think you get the idea. I had complaints and XP was good enough. It took six years for Microsof to develop a new consumer OS (XP to Vista) that I (and I'm sure a lot of other people) didn't care for. Six years is a log time to wait for a disappointment, and being a developer, I was quite upset with Microsoft for stymying progress (again, as I saw it).
I agree that stability was an issue and it felt mostly driver issues. I bought a brand new Dell in March 08 with Nvidia 8800GT graphics and although drivers were available, it took 6 months of driver updates before there was one available that did not randomly drop the system into low res mode requiring manually resizing the desktop. This was very annoyning and painted a very poor picture while I used my XP laptop with no issues.
Mind you that I bought my machine over a year after Vista general availibility and still had massive issues. Imagine the experience of people who bought in Jan 2007. Although today Vista is actually very good, after years of pain there was just no way to rescue the Vista image. The only way was to rename the system and cut the cord to the past... Windows 7 is very much Windows VIsta, but it is Vista 2009, not Vista 2007-2008.
I have IT pros at my company that wouldnt touch Vista with a 10 foot pole but are excited about Windows 7. I think that is good news for MS. Let the upgrade cycles begin...