A true, nation wide, WiMAX network would allow consumers to use their WiMAX device coast to coast, which is a positive development. However, it's not at all clear that a nation wide WiMAX network will truly embrace the Google / FCC ideal of "any application, any device" that's been proposed for the 700MHz spectrum. First, it's not clear at all what the distribution model for WiMAX devices will be. Given that the WiMAX device ecosystem will be small at first and subsidies will be required to get things moving, my bet will be that you wont be able to buy just any WiMAX device off the shelf and use it with the Sprint/Clearwire network. Rather, devices will be marketed and sold through Sprint and Clearwire, continuing the cellular device distribution model here in the US. In fact, because WiMAX is so new, it will take some time before other distribution models develop for consumers to purchase devices from. So much for the "any device" part of the model.
As for applications, 3G operators today (including Sprint) have placed usage restrictions that are, in part to protect the carriers' own content distribution mechanisms (they'd much rather you buy video clips from them then download them from YouTube) but are also designed to help protect the network. If some were to start trying to push 256kbps MP3 or video streams via their 3G connection, then users who are trying to check e-mail wouldn't be able to. Hence why applications like streaming video are generally forbidden in carriers' TOS.
While WiMAX is arguably more spectraly efficient and offers more bandwidth per users (Sprint is targeting about 2 - 4 Mbps as realistic performance), high bandwidth applications like high quality audio and video could saturate WiMAX cells in dense user environments. Will Sprint or Clearwire put caps or restrictions on what users can do? We don't know, but we can bet we'll see usage restrictions at some point. So much for the "any application" part of the model.