Simple improvement, that Jim himself mentioned in the video: Do not give game publishers absolute control over the user feedback forums on Steam. That way negative reviews cannot be silenced, as is happening now.
Of course that can mean more work for Valve, as someone has to mod those forums, and I guess that particular decision was a simple cost-cutting measure.
I'm all for the open-ness of Greenlight, and I think digital markets, once the audience learns and adapts, will naturally weed out the shady crap in favour of good products, as word of mouth has always done.... But only if the audience is given the tools to communicate with each other effectively.
As for people complaining that it'd be dangerous to have Valve decide what goes on Steam or what doesn't.... I agree, but thing is, they already have that control. Until Greenlight was around, you needed publisher backing to have your game on Steam, and, even then, Valve is perfectly capable of removing a game if they don't think fits with their Store (something, if I remember correctly, happened to a Greenlit pseudo-porn game).
In the end, what matters, in any market, is accountability. Same issue being discussed to death with all the "bribing" going around regarding game publishers. When someone releases a major fuckup, there should be consequences, and any and all systems that try to silence such consequences should be questioned heavily.