I'm new to this topic and haven't read all the previous posts, so I apologize if I'm repeating anything anyone else has said. One problematic item that came up for me was trekking poles. I forget which airline it was, but they didn't permit trekking poles for carry on. For this reason, I have made sure to buy poles that telescope short enough to fit inside my checked backpack, and to always have rubber tips for them, to protect my backpack from damage. Interestingly, airlines no longer care about you bringing lighters on as carry-on, but don't want them in checked luggage.
Something else on this topic: I recently was on a backpacking vacation in Iceland and flew Air Iceland for an internal flight after my first backpack. As expected, they wouldn't let me bring a fuel cannister on the plane (I had a partial left over from the first hike and thought I would ask). This is a common problem for them, confiscating fuel from backpackers, but they had a cool way of dealing with it: they told me that when I arrived in my destination of Reykjavik, I could go up to the Customer Relations counter and get a replacement fuel from among ones confiscated from flyers there. Very smart and considerate of them! Imagine if we could do that for all the pocket knives we lose to the TSA here in the US! smile