1. People mix up quality and their personal enjoyment all the time which makes the discussion about it very loaded. Quality is somewhat quantifiable, while enjoyment is purely personal. It's utter snobbishness to claim you only enjoy good-quality things, or to not be able to appreciate the quality of something you personally despise. At the same time, it's easy to understand why many people do not have that kind of distance towards media they consume – they affect our emotions and can become so important that we see every critique of our favourites as personal attacks against us.
2. Quality isn't objective, as Palas nicely explained, but I also hate the perspective that it's 100% reliant on context. We get to something like claiming that Twilight is a quality series because bland self-insert protagonists and shitty, sappy writing are not considered negative traits for a teenage romance novel (and I've seen a writer that I semi-respect spewing that). I think that the "does it do its job?" question is important, but shouldn't be the end of the discussion. Otherwise, once more, we lose all difference between something being of good quality and just being popular.
3. And as the same time, the pedantic approach of overanalysing pieces of media by some (even well-developed) set of technical standards, and creating some kind of "mean score" assessment is absolute nonsense too. A piece of media is never just a sum of its parts, so to say something meaningful about it you need to include contextual knowledge and subjective impressions about its effectiveness in whatever it tried to achieve.
So, to sum it all up... I'd argue that quality, if the word is meant to have any meaning at all, is a bit more objective than some people make it out to be – arts that develop over decades and centuries create their canons of good practices and techniques that are worth utilizing, and should be used as points of reference. But it's also not as important as some make it out to be – both because something doesn't have to be high-quality to provide entertainment and because high quality by itself doesn't create meaning. And that's good, because otherwise art and popculture would be awfully dull.