Alright, Nano-Tech.
So, the limiting factors I see* for nanotech are the following (pretty-much in order as I see it):
Materials - the nanotech will need a source of base material to work with, though because there is all sorts of material everywhere most assume this is not a problem. I say it is because I have a hard time believing that nanos will be able to turn any substance into any other substance. ("Hey, are stuck on some unforgiving, God-forsaken planet without a decent kitchen or cupboard? Well, never fear. With the patented Ronco NanoMeal
C just place a tablet on any old rock, and
Blammo! in minutes you have a piping hot steak dinner for two! With all the sides! Or, for those vegetarians out there, a wonderful vegetable soup complete with bowls and utensils! Doesn't that sound yummy?!?!") I think, that only limited molecular modification will be possible, so no turning inorganic materials into organic ones, and some substances may be just plain impossible to replicate this way. ("Oh, sorry captain, but we cannot reprogram the nanite warheads to create new dilithium crystals for the engines out of the broken bulkhead. The crystal matrix is too complex for the nanites.")
Software - the programing needed to tell the nanites what to make will be, I am sure, exceedingly complex limiting its usefulness. Using them to break things down will likely be the easiest/most common use for nanites. Basically, they can only do what they are told to do (unless you get into the "they evolved to be sentient" scenario, then they are still only doing what they are told to do, only that they are doing the telling themselves), and I am sure a certain percentage of them have to be used for the "brain", with this percentage going up the more complicated the programming.
Us - like the ego/hubris ideology I posed above, the makers of the nanotech will be the biggest limiting factor in what it will be able to do. Unless self driven, nanites will just sit there doing nothing until told to do something, and those orders will be limited by the fears and desires of the order givers and creators of the nanites. So, if we build in limitations that they are "hard-wired" to hold on to, some orders will not even be acknowledged (unless there is also a built in security feature to alert the nearest law enforcement agency that an illegal order was given), and certainly not followed. Sure, like any piece of technology they will likely be vulnerable to hacking, but I would assume there would be built in security features to make that very hard. Sort of like trying to hack the new NSA server farm; very, very hard. Perhaps made largely impossible due to "hard-wiring", meaning that you can hack a refrigerator all you want, but you cannot make it shoot someone because it isn't a gun. Because I believe most people are smart enough to see the danger that nanites can pose, they will (hopefully) build in some pretty-strong safety features.
*As I am not a super high-tech scientist, I am sure I am missing a lot here. [Though I could have sworn I had more to say before going to bed this morning.
] Hopefully, it is good enough to explain why nanotech is limited in a roleplaying game.