Numbers 2,6,8, and 10 are the big ones I see as common pitfalls. You'd be surprised how easy it is for people to lose their password and then give up, use tags that they make up themselves and therefore nobody sees their posts, don't follow anybody for some reason, and then just get discouraged because they don't "get it". Great post, good to see varying perspectives on this as time passes and the platform evolves with new users.