1. Conventional: Taken to represent trivial matters or trivial parts of a larger issue. Minor details are not considered necessary to understanding the "big picture" and are thus extraneous. Anything that is secondary to understanding the "point" of an issue can be considered to be minor details. Anything of little importance in general can also be referred to as minor details.
2. Sarcastic: A response which satirically downplays something important. Any usage which is used to satirically highlight a use given in definition 1.
Notes: This can be used either as a "subject" (grammatically) or as a response to another's comment.
Johnny: OMG, I lost my receipt from the gas station the other day!
Jane: Minor details, don't sweat it.
Johnny: I totally blew my math final!
Jane: Minor details. :P