If you've flown economy on any North American carrier in the last several years, you'll probably notice right away. I recently checked out an MD-80 (I had been upgraded to first) and found that the econo seats had almost as much leg room as the first class cabin, and the exit rows had even more room. They are still very narrow and crowded if you have someone sitting next to you, however.
You can tell which place have been done when you board. Look for a "More Room in Coach" plaque on the forward bulkhead (the wall between the passengers and the cockpit.)