WOW, everyone is seem to put up the quality vs quantity argument, let's be honest windows 8.1 or windows phone 8.1 lacks both quality and quantity of apps, how much quality apps are there on windows phone or windows 8.1, if you search through the market, you will see tons of fake or web wrapper apps, even first party company tends to deliver a web wrapper.
And besides apps, windows phone has tons of performance issue, like slow music app, resuming screen, no customizable lockscreen!, limited store payment option, and ofc the windows UI is horrible in some way.
People are complaining about apps, and blame devs or companies. But the core problem is MS itself, MS has a great history of making major changes to the OS, and that requires rewrite the app from ground again. Epic Games said the same thing in relate to their UE4 for windows phone or windows 8.1 app market, they wanna see what is the final form of the OS, No company wants to invest in a app and after few months they find out that they have to rewrite the whole thing again!!
And as for Android apps on WP, NO WAY!!!! The day windows phone can run android apps, most of windows phone supporter including me will sell their WP and choose Cyanogenmod.
I think the app gap will close when windows 10 will release. As developer will know that the are dealing with the real OS change, and not gonna change soon and break their apps. But i think ms should have less restriction regarding os, and gives the dev more access(ofc trusted devs), And more of all MS needs to prioritize their own platform.