I AM an expert on this, having worked opening day of phase 1, and every single busy season since, and I was there for the insane crowds that first summer.
We are definitely more prepared for phase 2 than we were for phase 1, because we absolutely did not expect those kinds of crowds, and flew by the seat of our pants the first year or so. Do I expect it to be terribly crowded this summer? Yes. Will opening day have lines lined up through CityWalk, just waiting to get in the park? Probably, who knows, we are expecting it. In summer of 2010, people sometimes waited longer to get in the stores than they did to get on the rides, or into the land. That would not surprise me at all for Diagon Alley.
My suggestion is the same as everyone else's-wait until mid- to late-August, or September, to visit. The crowds died down on phase 1 by mid-August, as the crowds do every summer no matter what, because the kids go back to school. After Labor Day, about the only people around are locals and folks with no kids, who can vacation whenever they want.