We did all four parks in one day on 6 April, starting with MK. We hit MK at 8:10 (EMH that day was 7 and then midnight to 3 a.m.), and had no issues at all until we left 9:40. We monorailed to EPCOT and ran into huge crowds--we literally crawled over planters and benches to get to the non-bag check entrance as the bag-check tables were backed up nearly 100 feet with throngs of people. At 10 a.m., the standby times at Test Track, Mission Space, and Soarin' were already well over an hour long. We FP'd Soarin' at 10:15, and the time we had to return was already 3:15 to 4:15.
We walked through EPCOT and headed to DHS where the crowds were also fairly large--90-minute waits for both RnR and ToT. Insteady, we FP'd Star Tours for 12:30 (45 minutes from that moment), so we did lunch at Backlot Express. The standby time at ST was an hour. After we rode ST, we bused to AK to ride EE, which had a standby wait time of 40 minutes. We were maybe five people from the door to get outside into the queue when the ride broke down. So, we bused back to EPCOT to wait for our Soarin' FP. In the interim, we rode Living with the Land (which had no wait) and enjoyed it. After that, we did Soarin' which by that time was a 2-hour standby wait. We got in under 10 minutes with the FP. Whew!
Thus, our experience with the parks on that particular day would have to be ranked as "very crowded." Other than the first hour and a half at MK, the crowds were large and wait times were long. However, as we ended the day on a high note (dinner at CG), it was still a great day!