1. What goes up, must come down. Sooner or later.
2. If you are inside watching TV and drinking a beer and you hear your neighbors out in their backyard calling for your cat, you need to go out and join them. Even if you think it's useless and that the cat will come down when it's damned good and ready. After all, it's YOUR cat. And if THEY'RE nice enough to be out there, YOU need to be out there as well. And no, you don't get to finish the beer first.
3. The reason that cats can get up a tree but can't get back down again is because of how their claws are shaped. Their claws come out from the paw and curve down and back, making them excellent for use as grappling hooks on the way up, but useless on the way down. The cat has to figure out that he needs to turn around and back down the tree. And cats don't like to back up.
4. If your cat needs to go to the bathroom badly enough, he WILL do so in mid-air. Stand back, is all I'm saying. If you thought getting crapped on by a bird was bad, well ........
5. See Number 1 above.