For me it was when I walked into Epcot on the first night of our honeymoon.
During the first day we went to Magic Kingdom but it was really croweded and we had no idea what we were doing back then so the only thing we went on was the Tiki Room because everything seemed to have a long wait time. We hated the Tiki Room and after walking around a bit we went back to our room to finish unpacking and to watch the Eagles game on TV. We freshened up and then went to Epcot as it got dark.. and woah.. walking into Epcot was amazing. The lights, colors, music, spaceship earth, etc were just an awesome sight to behold. As we walked into World Showcase the lights went down, the torches lit up, and the pre-illuminations music came on.. all very cool... and Illuminations really sealed the deal for me.. what a show.. and after Illuminations while we were walking towards the front of the park with the post-illuminations song playing was when I realized that this was a place that I would have to come back to many many more times.