Are you the type of museum-goer who studies every single painting, or do you stroll through the galleries and stop only at those that draw you in? I fall into the latter category. Perhaps I simply can't process the sensory overload if I partake of every magnificent work of art on display. It's too much. I need to focus only on the most interesting, or fabulous, or strange, or beautiful in each room. To enjoy the museum experience, I must pick and choose what I see.
When I'm reading, I expect the author to have already done this for me.
I don't want to see absolutely everything the author sees or knows, no matter how wonderful it may be. If there are numerous possible physical descriptions of a setting or character, I want the author to pick the one that does it best. Because, if you show me too much, I can't focus on any single thing. Nothing stands out.
To keep your readers engaged in the story, write your description as if you're a museum docent giving a short tour of a massive collection. What images must your reader see before leaving the page?
For example: The character you're describing may have many interesting features… a long braid of whisky-colored hair; mismatched Monopoly-piece earrings (a car and a thimble); slender, mosquito-bitten arms with scabs where she scratched them open; tight-fitting jeans covered in Sharpie drawings of butterflies and unicorns; a Thomas the Tank Engine umbrella; a length of floss tied in a bow around the tip of her finger; and the hiccups.
If you tell me that all at once my head will explode (it nearly did just now, writing it!). So, pick what you most want me to see, and show only that. Let your readers fill in a few blanks.
And another thing: On that museum tour that is your novel? Don't show your readers the floor and the walls. Don't waste time on detailed descriptions of everyday, background stuff… the asphalt of the parking garage, the sand of the beach, the blue of the sky UNLESS THERE'S SOMETHING UNUSUAL ABOUT IT. We don't need to know that the walls of the museum are beige and the floor is hardwood. Focus on the most fabulous piece of art in the room. (Unless the floor and walls happen to be splattered in blood. Then you might want to mention it.)