What does it take to damage an internally geared hub? What behavior can easily damage an internally geared hub (Shimano Nexus or Alfine for example)? 


can one jump off high (say 50cm) curbs?  
can it be damaged by shifting gears under too much torque?  
does water get in if it gets briefly submerged?  
are there any dos and donts, any habits one has to change after using derailleurs?


Edit/followup question: are hubs with fewer gears more durable in general?
Usually jumping curbs isn't a problem, as long as you're cognizant of not landing incredibly hard.  I would think that rough landings would damage the rim before the hub.

Most internal hubs wouldn't shift under torque at all.  Some have a system where they won't even try to shift if the mechanism is moving, others have a "sprung" system where it waits for a pause in pedaling and makes the shift then.  

Brief submersion shouldn't be a problem.  Prolonged would most likely cause big problems.  I don't know if shifting while submerged would exacerbate this...

The biggest factor I've seen in damage to an internal hub is using one that's slightly mis-tuned for a long time.  I'll have someone come in saying that it stopped shifting into third about a year ago, and now it won't shift into second...  By the time they bring it in to the shop, now it's a much bigger problem than it would have been a year ago.