With all due respect-- if a description is not available, why display a message like this?!
This is a useful message if you are debugging onebox. For techies like us, great, terrific.
But if you are a normal human being who just wants to post a link to some non-tech site and you don’t care about the internal workings of onebox, this message adds nothing except noise. I promise you, a vast majority of the people using Discourse (on sites about cars, software support, communities, audio equipment, etc.) don’t know or care what a “description tag” is, or why it matters, nor do they care if the link they posted “provided” it or not. All they want is their links to look pretty, period, full stop.
That said: yeah, I think we all agree that there’s no need to hobble onebox to work only on sites with a description tag-- especially when a description is not really necessary. A headline and photo not only look perfectly acceptable, but they are far better than nothing.
Neither Facebook nor Twitter require descriptions (and they never add a message like “This page provided no description tags”), and they have no problem working with just a photo and a headline to make a nice looking link. I see no reason that my favorite forward-thinking forum software can’t follow suit.