Like magazine editors, you could have a few stock phrases, like "I believe this is incorrect", "I believe this is not sufficiently important", "I believe this is not relevant to LessWrong", or "I believe this is poorly-written".
But if you don't have the time to explain, you surely don't have time to read an article and decide whether to move it to discussion.
Yesterday, someone moved one of my posts from Main to Discussion without telling me. Again.
I encourage the site administrators to show some basic courtesy to the posters who provide the content for the site. I believe this would be a better way of doing things:
1. Have a policy on what has to happen to move a post from Main to Discussion. Who can do it? How many admins are there who can do this? State this policy in a FAQ.
2. When you move a post from Main to Discussion, make a comment on the post saying you have done so and why you have done so.