Do you really need a license to develop Windows 8 metro apps?
Windows Store app development > New to Windows Store apps?
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Run Visual Studio to get a developer license. and mentions
To develop and test Windows Store apps, you need a developer license, which is free.
You can create this license when you start VS or there is also another way to get this license.
So, yes, you need a (free) license; I don't think it completely stops you from developing applications.
Even for third party ones not intended to be included in the Windows 8 store?
Yes, Metro apps are "Windows Store apps" and thus need the developer license in order to be made.
I thought the Metro was the future, and I understand why Microsoft would want to moderate what gets added to their store, but if you can't even post apps to third party sites then it doesn't seem all that great.
Yeah, that's the purpose of the store, being in control over what applications can and can't run. Compared to that, the internet is an unmoderated place which doesn't live up to the same expectations an application in the Windows 8 Store would.
A program that crashes, perform sluggish or do other stuff you might not want would not be able to enter Windows 8 Store; but can reach your computer if you download it from the web. I think you can share executables, but you really will not want to; you could just use the Windows 8 Store instead.
There hasn't really been a case of people being really denied from the Windows 8 Store; apart from some developers in the early days being unable to submit their applications for unclear reasons, which has since been approved so it shouldn't really form a problem nowadays...