Commonly, we do not question our most basic concepts. "Nature" and "culture" are two excellent examples. Sometimes, communication is actually easier this way; sometimes different understandings of the seemingly same thing we are talking about will lead us astray.

Where do we go with "sustainability?"

The Brundtland reports definition for sustainable development is always mentioned (use of resources so it does not diminish future generations'...). This has unfortunately been taken to suggest (as "weak sustainability") that it's quite enough not to diminish capital. So, as long as the money in the bank "grows" well, all's well.

Clearly, if we have any understanding of the world, ecology is a better starting point: sustainability as the preservation of our ecological life-support systems. In order to do so, if we accept that we do not know quite enough, we'll need to go towards caution. I.e., preserving rather too much than too little, not least because ecosystems are made up of species rather than abstractions.
In fact, in many cases, we would know quite a lot, "only" needed to use that knowledge rather than wishful thinking. (Fish stocks and commercial quotas come to mind.)
Also, having learnt that ecology and evolution go hand-in-hand, this needed to be oriented towards making it possible for ecosystems - species that make them up - to go on developing. This needs, e.g. migration corridors.

Most of that, though nice, sounds just like environmental preservation, however....
Secondly, it takes restoration, and in fact integration:
Human beings are a natural part of the planet, and sustainability is really about us.
Nature, let alone "the planet," goes on anyways, but we want to make humanity's possibilities better. (This alone, rather incidentally, is an argument that I think should become stronger, versus the "jobs-versus-owls"-like statements that still surface, e.g. to say that measures to combat climate change are too big a strain on "the economy.")
So, especially seeing how the use of ecosystems is commonly cheaper than any replacement (if that is even possible), we need to find and support ways in which our practices can work as, as parts of, and with, ecosystems. Not (necessarily) out of spiritual connections - although these shouldn't be cast aside - but for our self-interest...

[Yes, this is rather a stub. Feel free to let the article evolve....]