Telling details vs. clutter
Another one of the truisms about writing that you hear a lot is "the power of the telling detail." And it's quite true; a single specific detail at exactly the right time can do more to evoke a world or a mood than pages of description, even if we're talking about really well-written description.
In a sense, the definition of "well-written description" is "a collection of telling details." But what, exactly, is a telling detail? I'd say it's something that does double or triple duty; something that points to things beyond itself. Often, it's the unexpected or unique item or action that, just by existing in that place, at that time, says something or implies a whole lot of other things.
A telling detail grabs your attention. Too many of them, all piled up, become overwhelming. A single Lalique figurine displayed in the center of a marble table can be a dramatic statement; forty figurines covering the whole tabletop looks like a yard sale. The difficulty comes with where one draws the line. A grouping of two figurines may work just as well as the single one; three may be less dramatic but more symmetrical or more graceful; four…well, you get the idea. At some point, things go from "an attractive display of items" to "a mountain of clutter."
Say I have a character who walks into a bar. I haven't thought much about the bar, so in my head, they walk into a generic gray mist labeled "bar," with whatever default bar-stuff in it that my head comes up with: tables, bar stools, a counter, kegs of beer behind.
Now, I can describe all that and maybe even make it interesting, but it's all generic, default, just the stuff I'd expect to find in a bar (and so would a reader). What I want is the thing that's different. What's the one thing in this bar that, if I mention it, every reader who walks into this bar will instantly know they're in the place I'm talking about?
I could put a collection of antique beer mugs on a shelf over the bar, if they're strange enough or eye-catching enough. I could try to come up with unique tables or stools. But for this bar, in this story, what presents itself - the thing that instantly attracts my mental attention - is the mosaic depiction of a winery over the fireplace with the starburst of cracks in the corner where the stray bullet hit during a fight last year.
I'd call that cracked mosaic a very telling detail, because it not only what the mosaic looks like; it implies a lot about the bar. It's the sort of place where fights break out, where someone might pull a gun. It probably used to be more upscale (mosaics are fairly expensive), but it isn't any more - either the owners can't afford to fix the bullet hole or they haven't bothered, and either way, they're probably not doing a lot of maintenance on the rest of the bar, either.
The mosaic and the bullet hole don't have anything to do with the plot (at least, not right now, when I'm making them up. Maybe they'll turn out to be important later on, or maybe not. I don't have to know whether I'm going to use them later, or why they might be important. All I have to know is that this is something that grabbed my attention, that is a cool detail about this bar…and if I say "From where he stood, he could just make out the starburst of cracks where a bullet had hit the mosaic…" I can let the reader fill in the tables and stools and counter.
Or, I could come up with some more details to expand and modify the impression of the bar: the beer mugs that are lopsided amateur pottery with crooked smiley faces on the side; the giant Elvis-on-black-velvet paintings that are being used as curtains on the back windows; the dusty disco ball that's off-center in the ceiling; the jazz-rock version of "West Side Story" that's playing on the Muzak. After a bit, they start to meld into an overall impression of "old, odd, maybe a little tacky, maybe a little rough." If I go on too long, the impression will change again, to this-writer-talks-to-much-I'm-skippping-straight-to-the-action.
Exactly where the line is depends on the writer, the story, the style the writer has chosen, the reader, and maybe the phase of the moon. There isn't a clear-cut, unchanging rule for this stuff. It's like riding a bicycle - you can describe mass and force and momentum with equations, but what you really need is the feel for doing it.
And yes, in order to get that feel, you fall off a lot at first and skin your knees and bang your elbows. But that's what it takes for most of us to get that sense of balance. Once you have it, you don't have to think about what you're doing any more, unless you're navigating a particularly tricky stretch of road (and even then, it's not so much thinking about what you're doing and controlling every aspect as it is about paying attention and concentrating and keeping that feel of balance).

