The top contenders for "things to worry about" from the newbies in the hiking group, and in my backpacking class, are usually animals and bugs, and the women worry about random wild mountain men some of the time (being women we are usually socialized to expect that).

Experienced backpackers, or at least backpackers with a handful of trips under their belt, worry about getting hurt or dehydrated, or hypothermic. At least, the ones in my immediate acquaintance would say it - I hesitate to call it worry, they tend to be rather self assured about it. Having allowed myself (and being forced by circumstances one time) to slip into a state of mild to moderate dehydration, I'd say that's my biggest concern, simply because I seem to continue to get there despite all the sipping and water source research I do.

Actual statistics on searches implemented(see above) would indicate that getting lost is a bigger problem, and I tend to agree with that. I don't fear getting lost myself, but I am aware that despite ongoing refresher courses (SAR trainings slam you to the mat on compass skills regularly, and expect GPS proficiency as well as working with interpolators and triangulating skills) I am at risk as much as anyone.

Complacency can lead to mistakes, and some of my first search incidents were for veteran outdoorsmen (not just hikers, also hunters), so I try not to succumb to that "I'm an expert, I can handle it" assumption myself. Because I consider myself a well-researched and well-read, experienced backpacker - not an expert by any means. I think Wandering Daisy would qualify for expert status well before I would.