This will be interesting. I suspect that many people will not be able to upgrade right away as developers get their code caught up (or perhaps some items never will get updated.) For instance, many of the sites I have worked on rely on Solspace modules. None of those sites will be updated until Mitchell gets around to updating them. If he is extremely busy at the time, who knows how long that will take.
Even more of a pain will be modules that are created by solo developers are not available to the public. If I want to upgrade on a site with my own module then I will have to upgrade the module as well. This could create quite the backlog for people in this situation.
This may not be a concern as I am not one to rush to new versions just to have the latest and greatest. But the problem might be in support issues. The support policy of the forums is that users must be using the latest version. If users are not able to upgrade because they are waiting on updates from developers then will they still be able to receive support? Will the last 1.x EE release continue to be available for people who heavily rely on modules waiting to be updated? Will security vulnerabilities be fixed if found in 1.x?
This is probably more doomsday than will actually happen but it could create some problems. I also believe that these are acceptable issues for a better system and developers simply need to update or the modules discarded but the reality is that not everyone will be able to upgrade right away.
Also, will the developer preview be available to everyone?