I discussed a book with a friend, and he disliked that the author described a person who is dying shortly after and has no further effect on the story over a page or more. The same would go for things; if I describe a room, should I include things that have no importance for the story?
How many of these story-irrelevant descriptions are too much? Would they annoy the reader? And if I completely avoid this, would the story have to less 'flesh'?
Your friend is right. Don't describe things that aren't important for the story. However, keep in mind that things can be important for a variety of different reasons. You may want to describe to establish:
Strunk, White, King and the editor on the paper King interned at all agree, the key to successful writing is:
Omit Needless Words
As a further note Chekhov pointed out that if you place a loaded gun over the mantelpiece and nobody fires it before the end of the story it shouldn't ever even have been there. He was talking about plays but the principle is valid in all storytelling.
should I include things that have no importance for the story?
Everything you include in your text is by definition part of the story and will affect the reading of every other part: Every part is automatically important whether you want it to be or not.
The factors to take into account when considering whether to include something are:
Consider two extreme cases:
An unrelated character who is about to die is described at length. The reader's mind may now be seeded with thoughts of death, mortality, futility, tranquility, etc. (depending on the nature of the description) while they read the next part of the text, and their interpretation of it will be coloured by these themes.
A book that consists of a slim plot, riddled with tangential and meandering asides. This has the effect of exposing the reader to a vast field of disparate subjects. They are free to draw their own connections between topics and become very active in their interpretation of the text. They will judge the action of the main plot within the context of the wider world. In fact, the plot can even be so slim as to appear non-existent: The best example I can give of this is the film Koyaanisqatsi, but The Instruction Manual by Julio Cortázar and Life A User's Manual by Georges Perec are good examples in literature. You may not even have to try to make the deviations relevant to each other: the human mind will find stories in the most minimal stimulus, as shown by the experiments of Lev Kuleshev.
These illustrate that relevance is a complex and sometimes unintuitive issue. Further, relevance trumps interestingness. Consider The Princess Bride by William Goldman, ostensibly a rewriting of another book in which "tedious" parts of the "original" text form a sub-plot about its author.
Think, Stephen King vs Dean Koontz; Stephen King is big on describing things in deep detail in all his stories. Most of these things are relevent to the plot and some of them are relevent to the character. Dean Koontz on the other hand, has a tendency to describe things about characters that you either dont care about or are way too unnesissary, try reading the last Brother Odd book or By The Light of the Moon and really observe the way he discribes his characters, thier actions, thier history etc
Personal opinion; description is meant to place the reader in the same room with the character. They -- the reader -- need to be able to see, hear, smell, feel the same things as the character. If the character enters a new location the reader needs to be aware of their surroundings as well.
