Actually, there ARE items on every MEL which do "affect safety" - not directly, of course, but as a removal of a level of redundancy. That intermittent "L FADEC channel 2 fault" in post #2 won't stop the FADEC doing it's thing right now. But when a different fault (or maybe two faults) occurs in the FADEC, there'll be no more channel 2 to pick up the slack. (For instance)
Master MEl items are proposed and approved on the basis that the increased risk, due to the loss of redundant systems, is acceptable for the time limit of the MEL - its a risk/exposure thing. Which is why some stuff is on a very short time to fix, and some stuff basically could be broken for weeks and no-one will care.
(The ultimate example is the CDL, rather than the MEL, where because there is deemed to be NO safety impact, the item can be left unfixed essentially for the life of the aircraft. Then it becomes a purely commercial decision.)