A GPSA GPS needs batteries, needs the right maps loaded, needs to be able to get a lock, the batteries can run down... Mostly they're heavier, more expensive and more complicated then a map and compass, which you need to know how to use anyway as backup. Only once have I gotten disoriented enough that I felt lost and that was after I'd had a major nose bleed at altitude while bushwhacking. I'm pretty sure I could have backtracked from there without major issue, but luckily I was with Brewer, so I just made him navigate.
In general having a good map, knowing what your backstops are and knowing how to orient yourself have always been enough for me. The only reason I can think of that I'd carry a GPS is if I ended up in a SAR group that used exact coordinates heavily.
UnderwearIn civilization underwear makes a lot of sense. You change it daily so you don't have to wash your pants quite so often while still staying clean. That kind of breaks down when you're only washing all your clothing every two weeks.
Don't get me wrong, for short trips I'll still wear it out of habit, but for a longer trip like the AT or JMT a bit more airflow seems healthier. On the AT I brought two pairs of shorts. I wore one during the day, and the other at night. It was probably overkill, but I didn't want to wear the same piece of clothing continuously for months on end and I didn't want to get my sleeping bag dirty. I could see carrying undies - especially boxers for the "night shorts", but for the AT I really enjoyed having a pair of shorts that was easy to clean and hard to stain for hiking in during my period.
TP and potty trowelA sturdy stick can be a pretty effective digging implement. Likewise leaves, sticks, or rocks can do the wiping job pretty well. I really don't see the point carrying TP or a potty trowel, but if it's what matters to you then by all means take it along.
Water FilterYup, filtered water tastes better. I've also yet to be on a group trip where someone didn't ask to borrow my iodine 'cause the pump took too long. I carry Polar Pure, which is sadly not sold anymore, and really love it. Besides the weight savings of the filter itself I find that I'm much more willing to stop and refill my water because it's so fast.
What don't you carry?