this is a really hard question to answer for most people because everyone has a different idea of what "crowded" means. We tend to go at peak times (Christmas, Spring Break, Fourth of July) and rarely feel that the parks are "crowded" because we get there for RD, make use of FP, leave in the mid afternoon and head back later at night. We never end up waiting more than 20 minutes for anything even at peak times. If we were there from 11:00-5:00 every day I"m sure we would be miserable and never want to go back. Also, if we were to be there now, we would probably feel like it was completely empty. However, everyone has a different frame of reference and different expectations for "light" and "busy" times.
A 50 minute line for Splash or Thunder Mountain does not necessarily mean crowds were high. What was the wait for the non- E-tickets? The E-ticket rides are always going to have long waits in the middle of the day because they are so popular. The real difference between peak times and non-peak is that that 50 minute line for Splash, Thunder, Toy Story, Everest, etc will end up being double or triple that (yes- 100-150 minutes) and the lines for things like Small world will be at 50 minutes too.
It is also important to pick your park days wisely. If a park has early magic hour at 8:00, and you can't be there by 7:30, avoid that park and go somewhere else. The park will be jam packed with people who planned to be there at 8:00 but got a late start. There are lots of other tips and crowd calendars (personally, I like the easyWDW ones because he has written explanations... no offense touringplans, I still have a subscription to you guy!) can give you the best idea of how to choose parks.