The biggest barrier is compared to stream, garden just feels weird.
It's not like any of the ways in current prominence. It's not twitter, snapchat, facebook, or forums. Everday we rehearse the stream, it's become the dominant idiom.
I've taught blogging and I've taught this. This is much harder.
Another problem is that the payoffs are less immediate. With the stream you open the sluice and you're up and running on day one. Value is immediate, visceral.
With the Gardem, value is cumulative. There's a sort of Metcalfe's Law at work here -- the first thing you add is unconnected, and not valued. The 100th thing you add can connect to a hundred different things, and a hundred different things cann connect to it. And everything you connect to benefits from your previous connections.
All this is great, but you start to see the true power of this a year in. How do you sell a technology like that?
The Garden does not fit known business models on the web.
The web was not built with advertising in mind. The stream comes from the world of news, though. With the stream you build up a property and keep people coming back to see what's new.