Wait and see
Several publishers have already announced their plans and/or are working on things now. I believe that some of them have the GSL already, but I don't really know. The GSL is available to the rest of us tomorrow (Friday), or, at least, has been promised to be available.
I will wait and look at the license. I have several ideas for some 4e products but whether I try to publish them myself (I have yet to have products out but I am REALLY close now) or I propose them to a larger publisher is going to be dependent on the license and on what sort of payment I can get from someone else.
The rules system is pretty strong, though like others, it isn't *MY* idea of DnD, it is a good game and looks to be selling well (the FLGS has TONS of preorders). I think there are a number of gaps that can be filled by 3rd party publishers, depending on how open the license is. Whether it is "forgotten" races or "ignored" classes, WotC has left the door open for some quick products that will later be replaced by "official" material. As to how well those products will do, I think everyone is in wait and see mode.
Patrick