Easy of use - Unity is VERY EASY to use and just works (shaders just work drag and drop, its works in the web, on mac, PC, iPhone and Wii ).
I do love 3DGS its great, that's why I keep pushing them to add stuff, I think if they add the following:
1) Web player (coming) 2) Editor (coming) 3) Maybe add kick ass rendering stuff - better then other engines or same as AAA engines - see (http://www.leadwerks.com/engine_rendering.html)
The down side of Leadwerks is it a bit header to code for.