Description is one of those difficult things to get right, at least for me. I often find myself labouring over a small descriptive paragraph for hours, and still not being happy with the result - whereas a passage of dialogue or action can slip onto the page with minimal effort and remain almost unchanged throughout all the subsequent drafts.
The difficulty, I think, is not so much giving the description -- "the room had an altar and a stool, a wide arched ceiling, and three small clay leprechauns all in a row" as giving it in a way that reflects the viewpoint character's perception of the scene, the way he or she would naturally look upon it, how their eye would be drawn, and what their mind would do with the information received. "There were three clay leprechauns in the corner, like the type Uncle Herbert used to have at the foot of his bed. But there was no bed here, just a stool set before a marble altar, and a wide ceiling like the type the old Reformatory School had in its dining room."
The character here is filtering the information, noticing the most noticeable first, and only noticing the rest in terms of his/her efforts to understand the presence of the first. All the information is filtered through their own experience, their own sense of the world as they see it. This stops the description feeling arbitrary, like a list, and anchors it more firmly in the point-of-view of the protagonist.
The other thing about description is selectivity - working out what's worth mentioning and what's worth leaving out. When I was about twelve, and I used to write adventure stories, I would describe everything. "The man was about six feet tall, with blond hair and a square jaw, blue eyes and a droopy moustache. He was dressed in jeans and a polo-necked shirt, with a poka-dot hankie hanging out one pocket, and a pair of boots caked in mud from top to bottom. He had an unlit pipe in one hand, and a pistol in the other."
Way too much information for the reader to process, with the result that the scene is muddied, rather than painted. The trick, I think, is to pick two or three key details, again the sort of thing the protag would most likely notice, and let the reader fill in the gaps. If I were describing the above character, I might say something like this:
"He looked like one of those bush types, with a droopy moustache and square jaw, and boots caked in a thick crust of mud. Then I saw the pistol, hanging limply in his left hand. Thankfully, there was no hostility in his blue eyes, nothing to make me think he would raise it."
Gone is the hanky, the pipe, jeans, height, hair and shirt. I've still kept the pistol, boots, moustache, jaw and eyes. The hope is that the reader will have enough information to form a mental picture, and that they will fill in the gaps of any information missed. As the narrative continues, other details can be introduced, and the reader's picture is either confirmed or slightly adjusted (but hopefully never jarred). The overall result should hopefully be a clearer idea of scene, but even more importantly, a sense that the reader has an involvement in the story, that they are in some sense drawn in and caught up in the narrative process. In the end, that's what makes reading infinitely superior to TV or movies, where all the information is given to you on a plate. (Incidentally, I think that's why movies based on books are so frequently disappointing, because we feel as if our own private world has been reinterpreted by another, and not necessarily very well.)
These are some of the things I've picked up over the years, things that I strive to bring to bear in my own writing. I think that's probably why I find description so difficult, because I'm not simply writing a list, I'm trying to present information through the eyes of the POV character, while at the same time attempting to paint a vivid scene without spoon-feeding the reader or swamping them in detail. And, over all that, I'm trying to write words that flow well, are readable, and carry their weight.
Who'd be a writer, hey?