For me, the most concerning aspect of environmental sustainability is the lack of prioritizing long-term goals for health and wellbeing of the planet for humanity. No really - I know that's "HUGE" as far as concerns go; many would pick one specific aspect (air pollution, water pollution, etc.). All of my concerns come back to the source: pollution in general. It's why my background is in environmental science, and that I pursued an environmental management post-graduate degree. What can I say? I'm an 80's child when "Heal the World" was a big deal, the hole in the ozone layer was ready to kill us any minute, and recycling became widely accessible for residential communities. I truly have supported (and believe) that I (and you) with my choices, can save the world.
Environmental regulations started mid-1970s and only in the 1980s did the Environmental Protection Agency (EPA) become established to monitor those regulations on a federal level. Prior to the regulations, we invented some pretty "nasty" stuff currently affecting the environment. Without the regulations, corporations did what they wanted with the wastes, and effluents (buried drums, dumping in waterways without treatment, and releasing gases without limits). While regulations are helpful now, they are not the "savior" that one would hope they could be for the planet. Discharge is still in vast amounts for pollutants to our atmosphere, soils, and groundwater; under a permit or not, as an aggregate, the world is killing our ecosystems.
Even with the advent of pollution programs, the intervening years become more enamored with plastics, petroleum-based products, and a "disposable" society. Our interactions with consumer products; mostly around food and household goods (cleaners, toys, clothes) are a burden to the environment both from a production and a disposal standpoint. The air, water, and other finite resources on this Earth are all well beyond the tipping point. "An Inconvenient Truth" by Al Gore is an incredible read / presentation (if you want to YouTube it) to give you an idea of what I'm talking about.
There is irrefutable scientific proof that our consumer-based lifestyles have and are continuing to impact the Earth negatively. Not just that, but the chemicals and resulting pollutants we have created and their daily release into the atmosphere, waterways and injection into the ground have contributed to many medical ailments (especially cancer) in humans that are increasing at alarming rates.
So, for me, the longer we live in a society that does not prioritize environmental sustainability in relation to pollution, or recognize climate change as a result, I will be concerned over the kind of world I currently live and the kind my kids (and their kids will inherit.
I have hope though: If we value the individual contributions towards increasing the amount and quality of positive choices for our consumer-based lifestyles that are aimed at environmental sustainability, I (and you) can still save the world.