The main difference is overall size. There are 5 campaigns(4 in L4D)and most chapters are longer. Some of these campaigns take place partially or entirely in the daylight hours instead of night. The crescendo and finale events are also different. You no longer just camp in an area, gun down the horde then run to the escape vehicle, though there are still a couple of those. Most of them require you to trigger the event and then the event will not stop unless the survivors deactivate it with another button/switch. Sometimes the only way to stop the horde is to get into the safe room.