1) Major defects. An example of this would be a structural failure.
2) Things that lead to major defects. A small roof-flashing leak, for example.
3) Things that may hinder your ability to finance, legally occupy, or insure the home.
4) Safety hazards, such as an exposed, live buss bar at the electric panel.
Anything in these categories should be addressed. Often a serious problem can be corrected inexpensively to protect both life and property (especially in categories 2 and 4).
Most sellers are honest and are often surprised to learn of defects uncovered during an inspection. Realize that sellers are under no obligation to repair everything mentioned in the report. No home is perfect. Keep things in perspective. Don't kill your deal over things that don't matter. It is inappropriate to demand that a seller address deferred maintenance, conditions already listed on the seller's disclosure, or nit-picky items.