I think you just have to know what you personally are likely to use. For example, we rarely take OTC meds at our house. I couldn't tell you the last time I took a Tylenol or Advil, nor the last time either of my kids had anything. So, unless someone is actively sick, I don't take kids' meds along. I figure if I really HAVE to have it, I'll buy it either on-site or take time to go off-site for it. (And I have to REALLY need something to take time to go off-site; once I'm under the arch, I don't leave.) Likewise, with the suggestions here on the DIS, I've taken ziplock bags along -- never used a one, even though I use a lot of them at home. We don't often swim at WDW because we swim a lot at home, so keeping track of clothespins to hang up wet suits isn't a high priority for me. We have other routines for dirty clothes (I bring a plastic garbage bag) so I don't bring a pop-up hamper. My husband and I generally leave all our "stuff" in the same place every time are not like some people who are always looking for our keys, sunglasses and wallets, so I don't need to bring a bin to put that stuff in one place. I'm also not all that concerned about germs, so I don't bring Clorox wipes, handi-wipes, etc. I have had the same Shout wipe in my park backpack for at least 5 years. I bought a travel first-aid kit and haven't used the first item out of it as long as I've had it.
I do take a multi-arm power unit with me, so we can plug in all our devices in one spot. I keep a Sharpie in my car, so if I need one, I have access to it, and have needed it several times. As I said in my previous post, I carry scissors because I do find them to be useful for cutting tags off items, etc.
Everything sounds like a good idea when you read about it, but you just have to know what you personally are likely to use and what is worth the weight of transporting it.