I know a few people who had their checked luggage lost by the airline, and the aftermath was so painful that they basically refuse to check luggage ever again.
I am curious whether the boarding process has gotten significantly longer over time, and whether that costs airlines more money than they receive in baggage fees. I'm sure the answer is "not by a longshot", but still curious nonetheless.
I know this is why people don't check luggage, but knowing the possibility exists makes it possible to plan around. I don't often fly with more than a backpack, but when I do I always check.
Nothing I don't mind losing forever, nothing that can't be temporarily replaced. If bag gets delayed only have the airline deliver somewhere staffed to receive a package (like an office/hotel) and with plenty of time for the delivery to be made, or get it from the baggage office on the way out.
If I lose all my clothes in my luggage that isn't really a huge deal (I can buy new clothes), but it certainly puts a major damper on the trip.