I'm pretty sure the openness will cause quite a lot of issues.
On one hand, they'll need developers to be happy about the machine and to see potential sales in it.
On the other hand, it'll be easy as shit to just pirate everything and use that thing for emulation and pirated copies.
Also, if its as open as they say, once they open the floodgates you'll see TONS of shit appearing. XBLIG is curated, but look at all the utter trash on that service. You need curation, you need someone to look over the games that are being offered, to see if they actually work, to see if they're actually fun.
The experience needs to be streamlined and fun - if that's not the case and people will be flooded with bullshit the moment they see the dashboard, people will quickly forget about this console again.
For 99 bucks it's a no-brainer, but the real question is whether anyone will care about this thing 3-4 months after its release.