One reason people write long posts is that they want to anticipate objections (that may or may not be nit-picky). The reason for this is that having to modify your views as a result of the discussion is generally seen as a failure.

The solution to this is to see posting and discussion less as a competitive game the purpose of which is "winning" and more of a cooperative game the purpose of which is learning. That goes for both posters and commentators, obviously.

That said, you often need to explain difficult ideas in detail in order for others to understand, as noted below.