The problem with that term is that it's vague (what is "mechanical" in the context of a keyboard?), surprisingly subjective, and even misleading (in another thread where this came up, someone even came up with a definition that activation occurs before the end of travel, but a rubber dome over membrane board can do that if designed properly)!
So, the first thing we need to decide is, what do people talk about when they talk about a "mechanical keyboard" as something desirable? I'd guess that most count Cherry MX, Alps CM, and buckling spring as mechanical (with the caveat that buckling spring uses either capacitive sensing or membrane electrical switching, instead of the metal leaf contact that Cherry and Alps use). Also, quite a few people include Topre's rubber dome over capacitive sensing technology, even though "mechanical keyboard" is typically used to exclude rubber dome over membrane.
Once "mechanical" is decided... is there anything else we want to include? I'm not opposed to including even high quality rubber dome over membrane boards.
Then, once we decide what's included in our term, what adjective should we actually use? Mechanical is either too wide or narrow of scope, and too contentious, so we need something that can be clearly defined in a keyboard context (and using a definition that's intuitive based on its definition outside of a keyboard context).
Adjectives I'd suggest:
- High-quality (a bit on the subjective side, but meaningful)
- Premium (also subjective, and allows some quite crap boards that are being sold for a high price tag)
- High-end (see premium for problems)