I'm interested in both your list and the reasons why. Submitting your list before reading other's contributions would be preferred.
Optionally: who is (or isn't) successfully addressing them. Individuals, organizations, companies, governments, other. How and/or why not?
1. Water - Access to clean, safe water is an increasing concern in some highly populated areas. We will see a serious water issue in the US plains in 30 years, jeopardizing agriculture in that area.
2. Food - Currently we produce enough food but it's not evenly distributed. We also have some concerns over sustainability and environmental impact. Estimates say we can feed 8-12 billion people depending on advances in technology.
3. Waste Management - Feces and trash are a major concern. Gates is sponsoring research on toilets that don't require much water and don't require expensive treatment plants. Composting toilets could save a lot of water and possibly allow for cheaper, more water efficient greywater systems. This tech can also allow you to build in areas that skeptics were not traditionally possible. Compostable materials like cellophane, mycelium, or plastic substitute made from cassava or corn would help with the trash issue.
4. Housing/Land Management - I'm too dumb to explain this. Maybe Mark Twain said it best with his statement about buying land since they don't make it anymore. This especially plays into agricultural with the shrinking amount of land suitable for agriculture.
5. Education - No, I'm not talking about college. Basic logic/philosophy, personal finance, and civics need to be prioritized in school. To overcome the other issues we need a cohesive effort to make a meaningful impact.
Nuclear waste has been piling up since the first reactors went online and are stored onsite. The US designated Yucca mountain in Nevada as the long term storage site but due to NIMBY it has been in limbo meanwhile nuclear waste keeps growing.
Rising sea levels are estimated to displace many millions of people which will lead to refugee exodus with many eyeing Europe and the USA.
Increasing bitterness and division in US politics make it easier for demagogues who promise quick fixes to gain power. These charlatans may lead us to a path of self destruction.
Going a bit farther, people dehumanize other people, and then feel all right about treating them inhumanely. They do so based on race, gender, cis/trans status, politics, religion, sometimes even sports teams.
The reason is well human decency for a start. And it is hypocritical to look down on people that harm other people if we are doing it systematically.