The main reason Vista got a bad rep was partly because of initial hardware incompatibility/poor driver support. Which wasn't entirely Microsoft's fault, really, but it made me frustrated with Vista when I first used it.
The other reason was UAC. UAC isn't a bad feature now, but it was annoying going from XP to Vista when Vista was prompting you for seemingly every single task.
I told a lot of people (non-techie friends and family) not to get Vista at the time. I'm sure a lot of people here did the same. So when Vista was finally perfectly usable and stable (especially after SP1), even people who had no first-hand experience with it knew it by it's bad reputation. Yes, people are vulnerable to the power of suggestion, but it's not surprising given that none of us can be experts on everything. We often have to rely on intuition, out-of-date information, "expert" opinions, and potentially flawed memories of past experiences. As long as you, as an individual, question your own opinions and try to determine and counteract what biases you may have, you'll be able to make rational decisions... at least most of the time.
I don't think anyone who avoided Vista really missed out on anything, considering 7 was better and was mostly polished right from the beginning. The same can be said of 8.1 and 10.