The world list comes from a different server than the actual world itself. Thus, I suspect that folks spamming that server trying to get the list so that they can re-click to get onto their game server is causing that to have some performance issues.
The "waiting in queue" system seems to have been a much better model than simply "world is full, try back later" since it didn't lead to the overloading of the world list server(s). I hope they'll bring that back, but there may be some compelling reason to have made that change on the backend of which we aren't aware. Time will tell.