"Hmmm. It's nice enough. The action's too high. It needs a set-up."
Why do this? Because it is the go-to, old stand-by critique that has more of a chance of being correct than not and it makes you look like an "I've played so much- I'm such a pro- I've seen it all" authority. You haven't played one note on that instrument and already you know it just isn't quite right. Wow, you must know your stuff!
You are also very ignorant, arrogant, and just parroting what you have heard other guitarists say who probably don't know what they are talking about either.
The action, or height of the strings away from the fretboard, is adjustable (in most cases). You can set it anywhere you want. Most people prefer it on the low side, but that is not required. It is true that in the past, cheap beginner guitars often have had terribly high action. Does that mean a high-end one with high action is in need of adjustment? No. There are reasons why you would want the action high.
Now, you've told someone they need a set-up and you'd be surprised to hear they just had the guitar set just that way. Perhaps their role is rhythm guitarist in their band. They use a heavy strum technique and find when the action is low the strings contact the frets. Perhaps they have raised their pickups and the magnetic pull is dragging on those metal wires we call guitar strings. They may have raised the action to find a balance between ease of play and sustain. Perhaps they play other styles of music you didn't know about where they prefer high action and just want all their guitars uniform- acoustics and electrics.
Did you know anything about what they play or how they play before offering your opinion? No. But you'll argue that guitar manufacturers list a specific millimeter height for strings in the manual or that your guitar tech whom you constantly, mistakenly refer to as a luthier(!) told you that there is a specified height that they consistently use. Sure, there has to be an as-good-as-any general-use starting point. Have you ever sat in a new car in a showroom? The seat isn't set all the way forward or all the way back. It is in a generally good place for most people. But guess what? Your mechanic isn't going to tell you, "Oh no no no. This seat is too far forward. You need to set it back about three inches." You know why? Because it isn't his car and he isn't driving it. Just as that guitar you critiqued isn't yours and you aren't playing it.
What you might have said, if you even knew anything about not just guitars, but guitar playing is, "Your action is too high ...for me."
But you didn't because you wanted to look like a big-shot and...you don't know what you're talking about.
Some very high action examples:
Jazz: Freddie Green (on his road guitar as high as 1/2 inch off the 12th fret!)
Rock: J. Mascis (for whom Fender/Squier makes an artist model guitar (that they ship with the standard string height, of course))
Shredder: Michael Angelo Batio (who plays contrary to the belief shredding requires low-low action)