It's more the "hipster" mindset and attitude that's the problem. This attitude involves fashion and trends trumping all other considerations. Maybe that's okay when it comes to something like clothing. It isn't acceptable, however, when it comes to tools that are supposed to be productive, including software.
If you've got a better term than "hipster", I'm willing to consider using it. I'm just not aware of any other term that better describes the particular attitude that puts vanity, arrogance, ego, smugness and focus on appearance over everything else.
I don't think that these people, or more specifically their attitude and their approach toward software UI design, are "nebulous" nor "ill-defined". It's extremely easy to identify incidents involving them: Windows 8, GNOME 3, Firefox 4 and later, this new version of Google Maps, and the Slashdot beta website, among others.
There are some common traits we see with these cases:
1. They hijack an existing, well-established software product.
2. They throw out years, if not decades, of accumulated knowledge and experience.
3. They usually come in with little to no relevant experience themselves.
4. They consider appearance far more important than usability, efficiency or productivity.
5. They create a design that's obviously flawed in many different ways.
6. They refuse to accept or even acknowledge these many flaws in their designs, no matter how loudly long-time users point them out.
7. They release their changes into the wild, often forcing them upon users who absolutely abhor the changes.
8. Their design efforts drive away more users than they could ever hope to bring in.
The fourth and sixth points are the key ones here. They are the very essence of the "hipster" attitude, regardless of whether we're talking about clothing, food, software UI design, or pretty much anything else they're involved with.
90% of the complaints I see about things being bad now because of "fashion and trends trumping all other considerations" seem to just be people applying who are themselves putting fashions and trends above all other considerations -- its just that they found a fashion or trend that they favored several years ago and are upset that the rest of the world doesn't still favor that fashion/trend.
And that's not entirely unreasonable -- or necessarily detached from personal productivity. Something that is a common trend in a field where productivity is relevant is almost certainly also useful to some subset of the market. Likewise, compatibility with expectations driven by trends external to the field can make new fashions also more productive to the mass market, even if they aren't more productive to the subset that the prior fashion "clicked" with well on a deeper level.