First-Aid kits are the one I am most lax about checking dates. It is certain that meds are going to have an expiration and some other packaged items also have USE BY dates on them. A good idea to check them out before an outing.

Anything with a battery should also be checked for leakage or to ensure that the item works. I packed a small AAA flashight for a quick overnight to a friend's luxury cabin last week and when he said that T-storms frequently knock out the power and asked if I had a light, I said sure then showed it to him, the batteries were almost dead. It came from my ready packed daypack. Luckily he had several flashlights around the house.

Always check items for serviceability too. Knives oiled and sharp, matches still good, lighters working and so on. I kept a ferro rod in a pack for years in a small kit and one day opened it up and it had corroded to dust because the protective coating had been scraped off. Check it!