As I understand it, all "sideloaded" apps still have to go through the Windows Store. If it doesn't map to a free or trial app in the Windows Store, it won't install.
I believe it's similar to the Windows 8 sideloading process, which is that it has to be created by a developer and using a marketplace ID, but the app does not necessarily need to be published on the marketplace. Otherwise, you wouldn't be able to test your apps on real devices prior to releasing them in the wild.
I'm sure if you have a paid app and try to sideload it, you'll need to own it on the marketplace (otherwise piracy, etc), but for a free or unpublished app, I believe it should work fine.
That's true, since of course developers need to be able to test their own applications, but that's hardly comparable to letting ordinary users install third-party apps, as you can on desktop Linux/Mac/Windows and Android.
Except for Apple's higher developer fees, you might as well say that iOS allows sideloading, then.
If an ordinary WP user wants to install an app that hasn't been published, they're out of luck, whether or not they have an SD card.
The criticism was "I can't install apps from third party sources". There was never a requirement to make it particularly user friendly in the OP. If it's a Windows Store application, then XAP sideloading coincidentally is easy. If not, then it's still possible, even if it requires you to be more of a power user.
> http://www.windowsphone.com/en-us/how-to/wp8/apps/how-do-i-i...
As I understand it, all "sideloaded" apps still have to go through the Windows Store. If it doesn't map to a free or trial app in the Windows Store, it won't install.
http://www.windowscentral.com/install-apps-sd-card-tested-ou...
If that's the case, then this is not installing apps from an independent source.