It is all about perception

Just the mention of the name “Vista” was enough to scare off potential users. In one marketing campaign users rated Vista 4.4 out of 10 but gave a new operating system named “Mojave” 8.5 out of 10. Mojave was just Vista with a different name.
Now compare that with how Windows 7 was received. PC World named it one of the best products of 2009. CNET described Win7 as “what Vista should have been”, giving it 4.5 out of 5 stars. Techradar called it “the best Windows OS ever”.
Win7 must have been a major rewrite of Vista for this kind of turnaround, right? Nope. Windows 7 is basically Vista with a few focused, incremental enhancements such as a new Taskbar, transparent window borders, and some performance tweaks. The system requirements are the same as Vista, and the drivers are the same.

Source...

Bottom line for most people though, Vista was insanely buggy, especially x86, prior to SP1. 7, though nothing drastically different, was Vista simply improved and refined to the point it should have been prior to launch.

I really never had any problems with Vista. And I got it on Febr.1, 2007 which was the first availability day.

I never had any serious problems with Vista when I used it. I did wait a few months until I upgraded, but it was nothing to do with thinking Vista was crap because I hadn't tried it personally, so didn't pass any judgement. When I did finally upgrade to it due to having some constant problem with XP, I didn't have that problem with Vista.

I think a lot of people forget XP wasn't always the OS it is today and that did have it's fair share of problems when that was first launched too I remember my Dad installing it my PC when it was first released and I do recall having quite a few problems with it.

When I think of it, I have not had any major problems using any of my Windows products over the years.

3.1, 95, 98, 2000, XP, Vista, 7

I think with Windows7 we are really looking at Vista SP3. If you compare that to XP SP3, Vista did pretty well. The name change of Vista was only because it's legacy.

I bought Windows Vista once it was released and N E V E R had problems with it. Even today I get tremendously irritated when people come up to me to say that Vista sucks, is slow, nothing works right on it and shit like that. I had the personal pleasure of inviting some of these people to use it on a machine that was not as Windows 2000 / XP as theirs, and the result was obvious: I found myself talking to "other people" about Vista. The truth is one: human beings are averse to change, and Windows Vista has brought a lot and so sudden! Why I migrated to Windows 7? Because I love innovation! Windows Vista was an innovation, and I still love Vista!

I think what affected Vista was the lack of drivers available at the time it was released, or I should say the lack of cooperation on drivers from hardware manufacturers.

The early reports of hardware problems created an aura of bugginess from which the OS could never recover.

MS did an exemplary job in collecting drivers for the release copy of W7 (and spent a bundle doing it I have read) and I believe that has made all the difference (in perception).