Like other people said, a lot of systems were sold with sub-par specs and Vista at release had some sort of bug that would destroy filesystem speeds. I remember it taking over an hour to copy someone's word documents from the old hard drive to their new Vista machine. It was maddening.Vista worked well on modern systems (at the time). Its biggest issue was 3rd party hardware developers dragging their feet on writing drivers that worked well. Nvidia took a few months after Vista's release but they eventually got it right. Creative took years and (based on what I've seen) had lost a lot of their customer base due to this. Another problem was that Intel had a lot of old 9XX chipsets it needed to dump, so Intel and Microsoft came up with that confusing "Vista Capable/Ready" bullshit that fouled the marketplace with computers that were stated to support the OS but couldn't actually support the parts people wanted.
Another "problem" Vista had was the fact that it appeared to use a lot of system RAM. In fact, the OS placing as much data into RAM as it could was a good thing overall, but the common IT guy at the time didn't understand this and therefore labeled the OS a "resource hog". By the time people started to understand this, Vista's reputation was already all but ruined and Windows 7 was already in beta.