There are excellent posts throughout the thread--one that I especially like is by MEGoody: Patriotic citizens will take into account the consequences of exercising their right to criticize. Unpatriotic citizens don't care about the consequences, especially to others.
I think that is a key element of the definition of giving aid and comfort to our enemies.
I think another key element is that a patriotic citizen will be careful to be truthful, to develop arguments based on facts, evidence, and logic, and to avoid confusing fact and opinion. This requires considerable intellectual and emotional self-discipline and maturity.
General Lee certainly derived aid and comfort from those in the North who railed against the war without regard for the consequences of their speech on others.