Should unity set up a hosted Webplayer portal?
Somewhere where Unity developers could launch/introduce their games or put them up for licensing/union and people could play them and comment/rate/review them.
They could even have a public and private section for people who only want a limited group of people to see/playtest their games?
Yes. And also i doubt Unity will do that because maintaining it would costs a lot of time and money, i suppose.
Not necessarily if they use a cloud based provider combined with advertisement revenue a la Kongregate and Wooglie they could launch a service that would pay for itself. They would need to setup the service which would take time and money.
The point is that a Unity specific one would allow developers to share and exchange ideas and show off their talent, it could be tied into the Asset store so producers can show off their wares.
I think it would raise the profile of Unity, they could even look into providing a sales channel.