Why this forum use only one ad at the top and one link ad in the middle. Y cant they place more ads. I know too many ads is not good..
Non-members (and people who aren't signed in) actually see more AdSense units than that. If memory serves they see the half banner top right and two leaderboards wedged between thread posts. Don't worry - DP makes a fortune in AdSense revenue - thousands of dollars each day.
I really like DP because of the low no. of ads. There are few webmaster forums which are cluttered with ads and sometimes the ads annoy me.