The most useful reward: free map data
Rather than "useful" rewards as cherries and hammers, OpenStreetMap actually gives you back something real, which is the map data you and everyone else has put in. By having an open source license any one can use the data for anything for free, rather than only one company profiting from your free labour and selling it back expensively to you. ( and waze intends to sell its data. Taken from their forum: "our business case is based on selling the data accumulated, derived and analyzed which has real value.")
Because of this there have been many useful things come out of the OpenStreetMap data. Here a small and not complete list of things that are already possible with OSM data. E.g. you can put worldwide maps onto your Garmin handhelds for free ( http://wiki.openstreetmap.org/wiki/OSM_Map_On_Garmin ). You can use maps offline on your phone for free and save data and or roaming charges (e.g. offmaps for the iPhone http://www.offmaps.com/, GpsMid for java phones http://gpsmid.sourceforge.net/ ), use it for SatNav (e.g. via navit http://www.navit-project.org/ ), you can have maps in various different languages (e.g. http://cassini.toolserver.org/tile-browse/ ) or different rendering styles (e.g. cycle map http://www.opencyclemap.org/ or public transport http://www.öpnvkarte.de/ ). You can even create games from the data (e.g http://www.monopolycitystreets.com whos game (street) data is from OpenStreetMap, or for flight simulator scenery http://gallery.flightgear.org.uk/c1483094.html ) .
But as the data is free and not locked into a single commercial company , there are and will be many more innovative ideas that can be done with it.
I think that is the real reward (and the main difference between Waze and OpenStreetMap), not the fact that you get some hammers or cherries.