I agree with our reigning champion
There are often holes in a deck when it is first built, and posting it allows many people to find those holes and let you know about it. However, to truly gain from posting a deck, it requires 2 things.
1 - Know who to listen to and who not to. Some people will post suggestions who have no idea what they are talking about. You have to weed through that and find the truly great advice.
2 - Being teachable. It doesn't do you any good if you post a deck and Gabe tells you to change several things and then you respond back that they are all perfect already and you don't appreciate his criticism.
P.S. Also, posting a deck ahead of time makes it much less likely that your top secret combo that you the whole deck depends on, won't turn out to be illegal when you get to the big tournaments