Just to save some time: If you don't think that you're part of a society or if you only care about yourself, you can just skip this. If you have some semblance of humanity, read on. Also, this isn't about environmentalism, so you lose points if you mention a carbon footprint.
What is a sustainable life? It is a life that everyone could live without society collapsing from lack of production or safety or whatever. It is effectively a life that does its part. This doesn't mean the exact life you live. Society would collapse if we all became firemen, even though firemen are very important. But it means the general production and cost of life. For example, a farmer who grows just barely enough to feed his family is sustainable. However overall he's not really productive. The factory worker who makes a tractor which triples his food, he has created a net production, even though he was not directly involved in the food production. The manager who kept the plant running properly, ensuring that a hundred workers could build a hundred (thousand) tractors, while he makes nothing directly, his management assists the production of others and is therefore productive as well.
In other words, think of it as the combined answers to these questions: "Can the world live with you being you?" and "Can the world live without anyone being you?" The correct answers are yes and no, respectively. The first pretty much means you didn't kill the world. The second means that what you do is in some way essential. Perhaps the first question was redundant.
Let's apply this to WoW, since that's a common ground. At least I assume so.
Can everyone be DPS? No. Can a lot of people be DPS? Yes. It's not ideal to have 90% DPS, but we get by and ultimately they only hurt themselves. On the reverse not everyone can be a tank or healer and the shortage only hurts people who aren't tanks or healers. Well fine, it does sometime hurt tanks and healers in raids when they spend an hour looking for a last tank and end up disbanding.
On the economic front: Can everyone be a miner? No. But we need miners. The mineral output from weekly engineering tinkering of Ulduar bosses isn't very high. Let's look at scribes who take cheap herbs and turn them into those expensive cards and decks. Terrible right? Well no. WoW needs them. How about people who play the AH? They fail the test of if the world can live without them. Let's look at the middlemen that you sometimes see. They advertise for people to COD cheaper than average trade materials and then they resell them at a higher cost. They pass the test of the world surviving without them, since while the economy would not collapse if they were gone; they do provide a useful service. By always taking mats at a certain price, in effectively unlimited quantities, they encourage people to farm and sell mats who might otherwise be turned off by the riskiness of the AH or who aren't very good at figuring out prices.
The cheap mats buyer/seller is a bit like the manager. Neither produces anything directly. Production would continue without them. However they act as multipliers of sorts, increasing the production which is already there and through that they become net producers.
I recognize that my questions aren't perfect. The first one is too easy to pass while the second is too hard. Self-sufficient individuals are nothing overall, so the world can live without them, which puts them in the same category as people who are net losses. Perhaps a middle question such as "could the world survive if everyone did approximately what you do?" That seems too vague. But perhaps the zero people are bad just like the negative people. They are stagnant, producing no surplus to drive progress. But surely there are limits to progress. The planet can only support so much. Perhaps too much social sustainability leads to environmental unsustainability. That's apparently not a real word.
Go farm something, craft, tank and heal!
P.S. I forgot to add, while DPS are not unsustainable, perhaps the sub-2k DPS on Emalon are. Could you guess that I had to go through an excessive number of wipes a few nights ago?
P.P.S. I started out effectively saying not to make this into a hippie environmental discussion and look where I end up. I'm bad. :(