1. You can't check your hydration status. All that blue dye means I can't see what color my pee is. After a day of hiking it's nice to know if it's dark yellow (dehydrated) or very light yellow (hydrated), or clear (over-hydrated). Thankfully, some camp grounds/state parks have flush toilettes and once or twice a day you might try and visit them in order to check.
2. What shape is your poo? Unless there is a giant pile of other people's TP and fecal matter for your poo to land on, you don't know if what you just pooped is a nice long normal BM or not.
3. Splash point. Again, if there isn't a TP pile and you go number 2, you may be in for a wet surprise. Which is gross since it's not just water... nuff said.
4. They can be far away from your tent site, which means a long chilly walk in the middle of the night. Lucky for us, they were only 2 spots up from our site.
5. If you drop anything in it (camera, sunglasses, head-lamp, flash light, wallet, phone...) you don't want it back.
6. People who are on the shorter side (< 5' 4"), will not be able to place their feet flat on the floor of the port-a-john when they sit on the seat. Not a big issue for going numero uno. However, achieving the perfect 90 degree angle or better yet, squatting position for going numero dos is pretty much impossible. That is unless you balance on your sitz bones with your feet up on the door of the john. Now I know you've got some funny images in your brain. Guess I'll just have to purchase a squatty potty for our next camping trip, or home use.
7. Oh yhea...in the summer, they stink.
Sorry if that's TMI, but you know it's all true.