I had a set of really light wheels once. They were handbuilt and really nice but needed serious
truing every few hundred miles. The ones I ride now need only a touchup every couple thousand
miles.
There are a couple of ways to answer this question; the simplest
is to look at the manufacturer's warranty. Most of the really light stuff has a weight limit right on
the tag. My logic is - if they don't want to warrant it for a heavy rider then I don't want to be
depending on it. After that it's primarily an engineering design and metallurgy problem.
The way a manufacturer makes light wheels is to use less material; that ought to be
obvious. There are a limited number of spots in a wheel where weight can be cut once
you get past the really cheap wheels. The primary places to look for weight savings would
be in the rim and then the spoke nipples since those areas have a strong impact on
overall weight and on rotating mass. Rims are pretty well optimized (once you get past
the cheap wheels) so weight loss means less material means less strength. Additional
tempering or fancier alloys lead to brittle metals, so it is really tough to get ahead of that curve.
