In regards to what they should offer over the EDC, the EDC seems like a place where already created games, as well as examples, tutorials and engines, are placed. It is easy to browse through and has a standard format amongst other things. In many ways, it seems like a place well suited for static, completed and permanent content. Furthermore, if a user wants to play a nice game or look for a quality example, the EDC also provides a rating option.
EDC does not seem as suited to discuss works in progress as a "Works in Progress" forum would be, mostly because it is ordered (as least as far as I can tell) according to its submission date, not the latest comment, which means that works in progress with a lot of activity would not rise to the top of the listing. A forum for works in progress would be useful, because games can be presented, discussed and debugged, and if a work in progress does not work out, or the game is completed, or there is a period of inactivity, etc., then the work in progress thread will automatically go down from the first page of the forum.
As for the "Completed Games", the main difference from the EDC is that "Completed Games" would only have posts about newly created games, and games only, while the EDC contains games, examples, etc. that may have been created some time ago. But I suppose that this role is generally well handed by the EDC, especially given that EDC also provides a type to indicate whether it is a game, example, etc., and I presume that could be used to implement a filter to make it easier for users to browse and search through specific types.
While I will give you that "Completed Games" would likely not offer much over the EDC, I do believe that a "Works in Progress" forum is not covered well by the EDC (or any other part of the website) and would be very useful, especially in regards to getting and giving feedback on games.
In regards to promoting daydreaming on the forums, I am not quite certain I understand what you mean, but I am going to assume that you are talking about works in progress that have no working demos, or have no chance of getting completed, and the like. If so, I do see your point. Moderation could possibly be used, but that would only be partly effective, and take considerable amounts of work. What about having a "Open betas" forum instead of "Works in Progress"? That would clearly signal that the game should be in a very playable state and not far from completion, and it would still allow a lot of very useful feedback, even though the feedback in that forum would be limited to middle or late in the development of the game.