To be a good con man, you have to think like other people. Harry's not good at that.
A con man has to be good at playing to other people's superficiality.
Harry can't stand not asking questions. So he finds it hard to model others' willingness to not ask questions. Also, he's in love with getting the right answer, which makes it hard for his brain to think over wrong answers to offer people, even enemies. This is why Harry has no instinct for ambushes and active (lie-based, rather than silence-based) deceptions.
Someone like Harry can train to actively lie, deceive, ambush, etc. He'd do fine. But active deception is one of those social skills, like flirting, that mucks with the user's own cognition and so takes more practice than smart people initially think.
As for formation tactics, drilling people to act together is another less-questions-more-cooperation trait that Harry's never been good at. Chaos is definitely his right army name.
Well, I'd say there's a clear difference between ambushing (deception in tactical combat situations) and lying/manipulating (social deception in micro-situations).
The first requires way less self-deception: the requirement here is not control of vocal tone, facial expression and knowledge innuendo and social graces; no, here an understanding of which parts of the enemy forces the enemy appreciates, and which targets he would like to hit in your own army. The second calculations; terrain and so on are also logical advantages. So in effect it can easily be a...
EDIT: New discussion thread here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With two chapters recently the previous thread has very quickly reached 500 comments. The latest chapter as of 17th March 2012 is Ch. 79.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author's Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight, nine, ten.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: