This page is a list of stupid things people pack. Most of the time people over-pack because they over-estimate how long they're going to be gone for and under-estimate the availability of things outside their neighborhood.
Say you're going on vacation. The temptation is to pack everything and the kitchen sink. Have fun lugging around your useless possessions, half of which you won't use on the trip.
The greatest piece of wisdom I ever heard was this- “If you forget something, you can buy it there. If you can't buy it there, you probably don't need it, everybody else is getting by without it.”
The one exception to this is Sunscreen. Sunscreen is often times not used abroad and thus very expensive and hard to find. Always pack sunscreen.
Big Knives- The only reason you should have one is if you have a real defense need. You're going to build a shelter? Please. Use your tent or better yet, hike the fuck out. Keep a small sharp blade somewhere. I prefer a razor blade in my First Aid Kit.
Sewing Kit- Okay Betsy Ross, I'm sure you're going to repair that torn shirt in the woods. What's more likely? You're going to need to sit down and hand-stitch a huge tear, or you're just going to go home and buy a new shirt. You aren't going on the Oregon Trail here, we have towns with stores now.
Zippos and other Fire tools- Bring a Bic Lighter, nothing else. You know how many times my Bic has failed to light? None. How many times has my Zippo failed? The better question is, have I ever pulled a zippo out and had it light? No. Zippos blow. Flint and steel? Forget it. Too heavy.
More than 3L of water- You have to find water every day you're out. It weighs 8lbs per gallon, and that's what you have to plan on consuming per day. Want to plan a 3 day trip? Want to waste 24lbs in your pack on water? Of course not. Camp on water sources like everyone has done at every point in history. If you're confident about locations, you should never carry more than 2L at a time. Drink 1L in the morning, 2L for the day in the pack, then 1L at night when you make camp.
Flashlights- Anything handheld is useless. Ever heard of a headlamp? Get one. It's the single most useful pieces of gear you can have.