Our last trip was in April with DD4 and DS2 1/2. They both loved Epcot.
DS loved The Living Seas, Living with the land, Figment, the play area after Figment, Spaceship Earth, the fountains, the characters, O Canada and the Norway ride. We spent the whole day and he slept through most of World Showcase.
DD enjoyed all the stuff above, plus she was tall enough for Test Track and Soarin'. She did all the World Showcase Kidcot stops and really enjoyed that. We also did the princess lunch in Norway which was a big hit with both kids. DD wants to go to Epcot first on our next trip.
The previous time we went to Epcot was when DD was 18 months. We had no idea where anything was and ended up wasting most of our day wandering around and looking at the coots on a pond. We were so overwhelmed by the size of the place we ended up regretting wasting our day there. I think the key to Epcot fun is really researching what your family wants to do and then looking at maps to figure out where everything is. I couldn't believe what a wonderfully different experience Epcot was after we educated ourselves.