"The reality of the matter is that far, far more people will see the first few levels of your game than the last." Hmm I'm not too sure this is as true as it used to be, games have been getting shorter, and they also tend to have shallower learning curves nowadays. This is particularly prominant in HL2, IMHO it seems to have the exact same difficulty all the way though, with easier bits at the start and the end. The same applies for most of the current crop of 'narrative' FPSs.