ATP applicants need to have 1500 hours of flight experience—hence "1500 hour rule"—though there are exceptions if you've attended specific approved aeronautical schools or have military experience. Commercial applicants need only 250 hours. (Here too there are exceptions for certain training programs.) The standards on the flight tests are tighter for ATPs and the written exams are worlds apart.
As an instructor I can tell you the difference between a 250-hour pilot and a 1500-hour pilot can be enormous. Not always—some pilots have 1500 hours, and others have the same hour, 1500 times in a row—but there can be a huge difference.
This rule has been controversial though, not least of all because both pilots in the Colgan crash already had more than 1500 hours. And the FAA made so many changes after Colgan that it's been hard to tell which of those changes actually made the difference, and which, if any, were simply no-ops. Critics of the rule have especially focused on how many flight tests the Colgan captain failed, and how the response by the pilots to an aerodynamic stall was completely backwards. Aerodynamic stalls are something you learn in your first ten hours of training as a private pilot and drill constantly throughout your career, so to get that wrong at the air carrier level speaks to an incredible failure in training and evaluation.