Most Android devices can be opened AFAIK but there is no single supported way to do it, the process of rooting a device is more a hack in practice. This is why I consider Android a crippled platform.
And even when rooted, it's not your standard linux distribution, where you are used to get just about anything in there by installing a deb or a rpm package...
Until I see something concrete here, I will not hold my breath on the openness of Tizen. In the article you linked, Rasterman mentions that Tizen may be locked down due to company policy. If that does not happen, I am happy though.
As for Blackberry, that device is not linux, it's irrelevant to me.
Nah, you're just wrong. Fluid user interface is nice to look at and operate, but that's just surface.
What's important is that you have easy access to the real workings of the device, that you get things like access to the databases and call control from the command line using DBUS messages for example.
Maemo and Harmattan are far from fully open but they do get few things right; easy access to real command line, SW installation with real package management, ability to build and run your own kernel, documented interfaces to many internal interfaces.
As far as I have gathered information on Sailfish, opennes there is at least par with Maemo/Harmattan, propably more so. Even as I live and breathe Debian, I am ready to go RPM way if that's the price I need to pay.
I admit I might be in a minority but that does not mean my views are not relevant.
Well WP8 has an user experience. Do you consider it a success?