I'm curious to see how much of a return time "waiting area" will be needed. It's a small sample size, but the one time I experienced using a return time pass at Islands of Adventure--in Oct. 2010--about 4 months after their WWoHP opened--there wasn't a huge crowd waiting to enter the land. I don't remember the exact times, but my return time was something like 3:30pm. This time was about 4 hours from when I picked up the pass, so I went off to do other things in the park. When I came to the entrance of the land--admittedly about 10-15 min. past the start of my return time (e.g. 3:45), there were about 10 people in front of us in the queue to turn in your pass to the team member, and we walked through very quickly. There may have been about another 20-25 other guests hanging around the general vicinity--presumably waiting for their return time to open up, but there was no long queue or anything like that.
Again, this is a small sample size and the two parks are drastically different. But in my one experience, it didn't look like IoA needed a large waiting area for guests waiting to enter WWoHP. It'll be interesting to see how this plays out here in Hollywood with a completely different park layout.