If you really want to avoid crowds, then follow other advice given by others. FWIW, there are some downsides to going off-season, and I will list a few.
1. As you mention, attractions - DLR tries hard to keep these open during peak times, and closes them for refurb during off-season. Be aware that Sept is good time for low crowds, but that is when Haunted Mansion typically is taken down for several weeks to install holiday overlays. Small World goes down in Oct for the same reason. In Jan/Feb they both go down still again to take the overlays off.
2. Shows - During off-season shows and fireworks are scheduled less frequently, making them harder to see. During high season shows go every day, sometimes twice a day. So working in shows is easier.
3. Water rides - Off season usually means colder weather, and rides like Splash Mtn and Grizzley River Run are not as fun and often avoided for fear of getting wet. In summer, it is a relief to get wet.
4. Park Hours - In off season park hours are shorter. So while there are less people, there are less hours to see the park. In high season the hours are maxed out, and despite the crowds you can do a lot in the morning and evening if you stay at a close by hotel and take long afternoon breaks.
Here is a recent post I made that makes suggestions on dealing with high season at DLR:
http://www.disboards.com/showpost.php?p=12294563&postcount=7. During our trip last August 1-4 we never waited in line for a ride more than 20 minutes. Most of the time the lines were less than 10 minutes. If you are willing to do some planning, you can get the best of both worlds - namely, lots of rides and shows with short lines.