Sudden realization of sinister proportions
If you post your deck people will know about it.
What does this mean you ask?
Thank you captain obvious you say?
If people know your deck, then two things will happen:
1) Noobs will copy it and make it better or worse depending
2) Pros will adjust their decks and strategies to deal with your new deck
You then lose the "stupid mistakes" advantage that underdogs thrive on, and you spawn an innumerable amount of wannabe decks exactly like yours or very similar.