The trickiest part of writing, for me, is figuring out exactly how much is too much. Stories are often made or broken in the details, and the details also command how the reader reads the piece, how the piece translates in terms of atmosphere, and also commands the pace at which the reader can go. Higher word counts affect the pacing and structure of syntax depending on the density of words to paragraphs, and also factor into how penetrable the prose is to varying reading skill levels. Word choice and placement also factor into how pressing the prose becomes, by modifying how the reader is meant to read each sentence, and how those sentences interact within the paragraph. Connotation states not only how the character is feeling, but also what sorts of sights a character or atmosphere will transmit, and that affects the feeling of the reader.
In short, it's a terribly complicated and overly complex series of variables that will on a very small level effect how a piece is taken at any given step. Granted, this is taking word theory to an entirely too precise level to even be manageable, but it doesn't hurt to have a relative idea of this in mind when approaching how a part of a story is going to be laid out. The end result becomes its own monster, once the very finite processes are to be addressed individually.
Although, a point of some debate I often have with other writers, and avid readers, is in the details. Specifically, how much is too much? Seeing as detail density has a subtle effect on the pace and atmosphere, but it also has a direct control on how much the reader can see and interact with.
In my experience, details exist either as a form of setting formation, or as a form of characterization. My experience stems almost exclusively from the first person, so it tends to favor the detail level of the character in question. A narrator like Psych's Shawn Spencer will often key into as many details as possible, covering both a wide range of rough items while being very specific about certain key details. The opposite would be a narrator like The Doctor, from Doctor Who, who would likely key into the most pointless items in the room, and find a use for them in the most haphazard and unlikely way possible. This serves not only for scene setting, but also a very finite point of characterization.
Though that's only a small part of how narration and detail play a role within prose. Another outlook is on adjective and adverbs - descriptor words. These could season a single word or pepper an entire sentence, each piece having both specific and broad changes throughout the narrative, on individual and universal levels.
Although the question still remains. Over-detail a piece, and it could cause a number of problems. Purple prose is always a likely outcome, either reaching professional infamy or simply an interesting concept selling poorly due to format. For example, Eragon for the former, and Dune for the latter. Under-detail a piece, and it will often become either an airplane book, or too unfulfilling for the reader. Although acceptable levels will vary drastically from reader to reader.
In my opinion, the level of detail should be unique to the narrator, and beyond that, establish as loose a setting as possible. Any particular time a setting is filled in too heavily, it will often domineer the bulk of the text, commanding more attention than necessary, and leaving a relative short list for the actual goings-on, which is usually what I find preferable to read. That means establishing a scene through atmospheric particulars (colors, lighting, sounds, nearby moods), and rough overviews (diner, public park, cafe), and leaving the rest to the reader's imagination. Any more I find begins to become a little smothering, and any less is only used whenever the tone is intentionally left ambiguous and unstated.
The more detailed scenes are specifically designed to provide key details for future reference and later plot points. This could include a character noticing a heavy object is missing from a haunted house, which will later be dropped on/around said character, or simply taking notice that there is a flashlight in the cabinet for when the power goes out later. It shows forethought on the part of the character, and reduces the feeling of deus ex machina when the item comes into play later.
Just a some food for thought.