Count the number of mp3 players that's compatible with iTunes.
Well, that would be one. But if you own another player, pretty much every song you can buy on iTunes, you can buy somewhere else in a format that'll play on whatever MP3 player you happen to own. So in the short run, at least, this restriction isn't a problem for anybody.
But I think I can see a problem in the long run. If you own an iPod, and you buy 1000 songs from Apple, and then your iPod breaks, you are locked into buying another iPod. Again, most current iPod owners won't mind that at all. But some might want to switch brands--and can't, without re-buying all those songs. And if some other manufacturer comes up with a really great innovation, those locked into JobsWorld won't be able to take advantage of it.
Of course, smart iPod owners can avoid this by buying their music from somewhere other than iTunes.