Here is my take:
For WDW forums DIS is king. AllEars has lots of good info too but I have no idea if they have message boards.
For DLR things are mixed. DIS has lots of good, solid info on DLR through the menus at the top. However, MiceAge and MousePlanet have sources inside Disney they use to get information so are usually more able to get the scoop on upcoming news.
MiceChat (part of MiceAge) and MousePad (part of MousePlanet) have strong message boards and lots of folks who have expertise on DLR. More than I do, that is for sure. However, IMO they tend to be more focused on day trip visits and visiting with friends. Also they tend to be very ignorant about WDW although they would not admit it. Plus they have a very strong DLR bias over WDW.
DIS DLR draws more folks on multi-day visits and visits with family and kids. DIS also has a better mix of WDW vets with a more balanced perspective on Disney, IMO. And this forum tends to draw in WDW vets who hang out on DIS WDW forums who find it easier to come here than find another website. Plus, at the risk of tooting my own horn

, I think DIS DLR has better information for WDW vets and is much better able to help WDW vets plan a DLR trip.
Finally, I used to post on MiceChat and MousePlanet and now only do rarely. They are just not as friendly there. And frankly, I do not like most of their threads. There are way too many "armchair imagineering" threads for me. I cannot understand why people would spend time giving their ideas for new rides at DLR. Who cares?
DIS has the best DLR Paris forum, IMO, with lots of Europeans (many British) with WDW experience. Forums for Tokyo Disney are very weak across the board. MC was the best I found but still very skimpy.
Anyways, there are a few other Disney forums but DIS, MC and MP are the best ones.