According to Guns, Germs, and Steel, what were the major factors limiting crop domestication among Native Americans in the Eastern US?

The answer to this can be found towards the end of Chapter
8.  Basically, Diamond argues that the plant species native to Eastern North America
were simply not very suitable for domestication.  They had advantages, but these
advantages were outweighed by their disadvantages.

Diamond
says that there were seven crops that could be domesticated in this area.  They were
squash, sunflowers, sumpweed, goosefoot, knowtweed, maygrass, and little barley.  All of
these were good plants in terms of nutritional value.  However, they all had major
problems.  The last four all had seeds that were much smaller than crops like wheat or
barley that existed in other areas of the world.  Sumpweed also makes people very
allergic and can cause skin irritation.

So, the only grains
that Native Americans in the East had were not very good for domestication.  There were
no really good grains and so the only crops that were really valuable were sunflowers
and squash, which are not enough to be the basis for an agricultural economy.  This was
the major fact limiting crop domestication in this area.