No doubt, manufacturer's building units with 1GB of RAM to run Vista and using horrible processors didn't help, but Vista was a sluggish beast even with decent hardware. I've seen a handful of units that ran it really well, but those are rare. In addition, it got a bad reputation because of all of the folks installing old printer software and causing BSODs. On that note:
1) Microsoft should not have allowed the installation since it could clearly tell that the software wasn't compatible.
2) End users shouldn't have proceeded with the install when it JUST told you that it wasn't compatible.
It's a 50/50 fault there, but it could have been done better overall, I think. I truly believe that Microsoft was in too big of a rush to release the new OS.