Great neighborhood, near sun! Lots of beachfront property!  

^{Photo Credit}

• Continents  4, 5, 6, or 7 (depends on who’s counting)
• Oceans 1, 3, 4, or 5 (depends on who’s counting)
• Existing Occupants Human: 6.74 billion  (Nov. 2008) ^[1] (Also some animals, plants, protazoa, bacteria, molds, yeasts, viruses…)
• Future Human Occupancy 7 billion by 2013; 9.2 billion^[2] by 2050
• Owned By* All habitable units but 1 (in Antarctica) owned and occupied by 201 “countries”

*(Ownership disputes tend to be lengthy and explosive)

I’m sure I’m not the first person to postulate that we, the human race are in fact the worst renters in the known universe. We are those people.

• The ones who move into a home somebody else owns, who have no particular reason to treat it kindly (or even think about the notion of “treating it kindly"), and so proceed to shred the place.
• The ones who punch holes in the wall, let the toilet overflow without mopping up, let a loose screw become a broken cabinet, tear up the wallpaper, and spill many kinds of liquids, including bodily fluids, on the carpet. 
• The ones who think nothing of putting a broken-down car on blocks in the front yard, never mowing the lawn, filling up the porch with bags of rotting garbage (dude, I wasn’t up early enough to haul it to the curb to get collected, man!), having loud drunken parties with their shiftless, filthy friends.
• The ones who never pick up the dogshit or empty the cat box.

Wait.  

Actually, no—come to think of it, we don’t even pay rent.

We essentially just flopped here, and now we’re chopping up the floor and furniture and the walls to burn those for heat.

We’re Squatters

Here: This is fun. The history of the planet, using a roll of toilet paper as the timeline. 

   ^{Source: www.worsleyschool.net/science/files/toiletpaper/history.html}

Huh...

Then, a mere couple hundred years ago (kids, that’s just two or three cotton-soft fibers in toilet-paper time) mechanical super-geniuses invented The Industrial Revolution and modern medicine at the same time!

Since then? Quicker than you can say “The smell of burning fossil fuels must make us wanna reproduce like rabbits,” we started to make a whole lot more of us.  

Looks Like The Stock Market, 2001-Sept. 2008, Doesn't It?

And we all know how that's turned out.

So, here’s where we are, at least as I see it. 

• Our flophouse is currently in a desperate state of disrepair.
• The floorboards are mostly gone (we’ve burned them). There aren’t many more where those came from.
• The walls have just about as many holes as we can make without bringing the roof down on our heads.
• The water’s about to be shut off.
• We’re awash in mouths to feed (even though we often don't feed them, and aren't particularly concerned that apparently 64,000 people starve to death every day)
• Once the remaining mouths grow up, they’re going to start rubbing their naughty bits together and making even more mouths to feed…

And we have nowhere else to go.  

^{(If I were the owner of this property, by the way, I’d be mightily pissed. Would those who believe in The Great Cosmic Property Owner please start working on what exactly we’re going to say if He ever comes around to check up on the place?)} 

Which brings me, in a roundabout way, finally, to the big question.

Sustainability. 

I don’t know about you, but to me, this whole picture looks unsustainable. 

I mean, sure, we can (actually, we must) develop viable renewable options for human energy demands (because, if we don’t, we’re dead. Literally dead. At least, a few tens of millions of us will be). The number of humans on the planet create an unimaginable demand for clean water, nutritious food*, and the minimal protections of clothing and shelter. (That comes way before things like cars, paperclips, iPods, and flat-screen High-Def TVs, mind you).

We need energy for all those things.

• Fossil fuels (mineral oils, burnable minerals, natural gas, nuclear fuel)

• Plant oils
• Animal fats (I know, eyuwww--but, well, it’s true)
• Solar energy
• Wind energy
• Geothermal energy
• Oceanic wave energy

And yeah, it’d be sort of cool to figure out a way to move ourselves from one place to another (an activity humans seem to do a lot of) in vehicles powered by the sun, or electricity generated by wind farms.  

But it seems to me the problem is still there.

We’re still squatters in a flophouse

• We’re still not paying rent (i.e., engaging in a fair bit of maintenance and upkeep around the place).
• We’re still treating our only home as something to use up and throw away.
• We’re still breeding like vermin in a garbage dump.
• We’re still ripping giant holes in the ground to dig out rocks that we can melt to make into things like cars and iPods and flat-screen LCD TVs.
• Alternatively, we’re digging out rocks so we can cut them up and polish them to make sparkly things that some of us, for some reason, like to wear.
• We give and withhold food, water, shelter, and clothing to/from each other based on little bits of paper that used to symbolize some of the rocks dug up from the ground. (Nowadays they symbolize something far less tangible.)
• The current system by which we give and withhold food, water, shelter, and/or clothing from each other requires us to engage in all of the above, in order to "buy” and “sell.”
• While I wrote that last sentence, another 20,000 people rubbed their naughty bits together and conceived a new mouth to feed.
• We tend to kill each other over food, water, shelter, clothing, and arguments about what the Landlord’s name is, what he looked like the last time anybody saw him, and what he said about The Rules. Not that anybody would follow them anyway.

My Disturbing Conclusion: We, the human population of the third planet from the sun in this particular solar system, appear to be, in the long run, catastrophically unsustainable. 

Bummer.  

Image Source 

POSTSCRIPT (But Seriously, Folks):  In other words, my short answer to the sponsor's question is, "There is no one issue that is 'most important.' There are a host of interlocking issues that make up "sustainability" and to address them piecemeal = Massive Failure.

Were I to be appointed Sustainability Czar, I would push for three global initiatives to tackle the following issues in tandem: 1) Reneweable energy, 2) Overpropagation, and 3) Carrying Capacity."

Ultimately, though, basing human society on monetized manufacturing and consumption will probably be the death of us, because it incentivizes self-destructive habits in re: our planetary home, rewards selfishness, punishes altruism, and privileges tribalism (which will probably be the other death of us).