First of all, you need to have a dedicated GPS device (not many normal people do have one). Then you have to go to some not yet mapped area and collect the needed data. IMHO, this is the best part - a kind of discovery of unknown lands. However, at least for me, the fun part stops here - the rest is work. When you have your GPS log you have to return home, connect your GPS device to a computer, transmit the data and convert it into the format accepted by OSM (most people do not care about formats, and with basic command line conversion tools probably they will not be able to accomplish this operation). Next you have to edit the map. In general, this could be a fun part but currently too complex and time consuming: you create every road manually by connecting points which you add yourself and use the GPS log as a mere visual hint to where put your points.
What is my proposal? I would like to put the complete map creation process into a single device - the iPhone - the device that many people will have anyway. No boring PC homework anymore - just discover the world and make the map at the same time (and after you done post it to the OSM server instantly). I have chosen the iPhone because I personally like it, because I beleive that its user friendliness would be a great benifit for the application, and even more important, I like to develop for it (in contrast to e.g. Windows Mobile). Then I would like to try to apply some advanced statistical stuff to create ways directly from the GPS data. Being used by robots to automatically generate maps for high precision navigation, I beleive, these methods should work well for OSM.
Why not a web application? Well, web applications work good only until you have network coverage (and in some places, e.g. for cycling maps, maybe you don't). Furthermore, native applications provide better flexibility and performance - very important thing for we are targeting to a 'fun' application.