Sun giveth, sun taketh away

The world is already unstable. The economy is tanking, unemployment rises, public discontent as well. A badly timed solar flare knocks out a large proportion of telecommunication satellites and terrestrial power distribution networks; cascades of failures (redundancy? What redundancy?) blackout entire countries for days, some areas for weeks. Cities, now full of people and with disabled infrastructure, in few days break into chaos. Global economy, losing its main financial centers and with most of its communication lines severed, undergoes cascade failure; some of the centers recover but too late to resume business as usual. Latent tensions between several nations escalate into regional wars, as leaders fueled with ambitions and wishful interpretation of incomplete intelligence seize perceived opportunities, and cascade out of control like a bar brawl; limited but sufficiently intense nuclear exchanges (Pakistan vs India, perhaps Israel joining the fray and nuking somebody too? Can China get involved? ) may take place and cause a runaway in already destabilized weather patterns. Biological and chemical weapons may be used regionally as poor man's nukes. Nuclear winter sets in, with devastating results for agriculture, resulting in widespread famines and further intensifying conflicts, adding food and water into motivations for fighting.

Couple years later, the world is a patchwork of relatively working city-state regions, Somalia-like failed states, glass parking lots, and deserted shells of cities. Technological development continues in enclaves with resources, with much larger allowances for risk and side effects as the stakes are higher now. Genetic/technological modifications of people aren't uncommon, mostly for military/paramilitary purposes. Areas low on technology are rich on violence, areas rich on technology are under constant surveillance as the power structures are attempting to reestablish themselves and prevent the always threatening order collapse.

Good, bad, that depends on what side you're on. Each is just attempting to survive instead of the other one.

There should be a few hinge points in the history on which the breakdown depends, which can be interrupted - whether by talking a person out of some action, blowing up some system in advance in order to prevent its use to escalate a conflict or to prevent it yielding data used to start one, etc.; a few power lines may be severed, e.g. by blowing up a couple high voltage pylons, to force a power station offline during a critical time to prevent more serious solar flare related damage to its systems, speeding up region's recovery to avoid cascading into a failed state. A not-exactly-important person may have to be killed before gaining importance. An information may be given at right time to right person to alter their decision. A phone switchboard may have to be blown up or forced to fail other way so a crucial communication does not happen. A reactor may be forced offline so the transformers are not energized when the flare comes. A redundant power line has to be severed so during the following cascade failure a region gets disconnected from the grid (so called islandization) instead of being pulled down like the others. Details like these, relatively small changes in a number of predetermined points in space and time.

The hero may not have any really special superpowers; perhaps just eidetic memory, talent for handling high-complexity systems, and ability to see their hinge points where small changes have disproportionally larger effects.

Coup by addiction

Okay, this is a little bit corny but hopefully this avoids repeating what has been done in other doomsday films:

HOW THE WORLD ENDS:

A rather evil group of people has gradually inserted some kind of addictive substance into most of the world's urban water supplies. People begin to become dependent on water supplies that contain it, and drink more and more of the stuff. The substance is very hard to detect, so people don't know they're drinking it, and scientists don't notice it either until it's too late.

This addictive substance has another effect: it gradually kills the person who drinks it. Anyone who drinks it for more than a few years will die a horrible death, but they cannot stop drinking it because they're addicted.

By the time this substance is detected it is too late, most of the population is hooked and on its way to an early grave.

The villains then decide to sell doses of their extremely limited and difficult-to-manufacture antidote to the highest bidders. They only have 10,000 doses for the entire world, so they can demand quite a high price. Then when most of the world is dead, this group of evil people can use the enormous resources they got from selling the antidote to rule the survivors.

WHAT THE HERO HAS TO DO:

The time traveller featured in the film has to stop this plan from happening. They have to stop the water supply being poisoned, and deal with the people who are already getting addicted to the substance.

However, because the substance is difficult to detect in water, everyone thinks the time traveller is a complete loony and he has a real struggle to convince people to take action against the plot.

Environmental disaster

He's from a future when there are no more fish in the sea, because they've been fished to exhaustion. The oceans have turned poisonous, because there are no fish to consume and sift the waste matter. BTW, according to some leading marine biologists, this is is likely by 2048 at our current levels of fishing.

The seas and oceans have developed deadly viruses that have the potential to wipe out entire communities within days. The oceans are covered in an algae-like sludge which releases clouds of spores that, when they come into contact with animal or human skin, eat into the flesh like a plague.

The bad guys are the international mega-corporations, who have been sponsoring the fishing to extinction of numerous species, so they can store the fish in huge guarded freezer units and sell the fish at extortionate prices to pharmaceutical companies and rich fish-connoiseurs.

The good guys are a series of private individuals who have developed genetically modified fish spores from DNA, capable of re-seeding the oceans with a small variety of hybrid fish species in a period of 20 years, producing enough fish to consume the algae and clean the oceans. There are no more fishing fleets left, so the chances of extinguishing this new breed are remote.

The bad guys - the corporations - have armies and agents hired to stop the environmentalists and silence their protests. The protagonist comes from a time when the average human being doesn't even know what a fish is, let alone remembers an ocean full of them. People are misinformed about the oceans of the past and are educated to believe that fish occur rarely in nature and would never survive the oceans.

The future struggle is to get a sample of the spawn into the sea without the bad guys and the governments intervening. The today-struggle of the protagonist is to inspire environmentalists to sabotage fishing fleets and to stop the world from heading straight into environmental catastrophe.

The plot can be thickened by employing the need of some kind of special equipment or device to "charge" the fish spawn as they're released into the sea, without which the new breed will fail.

2011 is quite Soon, But whatever

As I said 2011 is soon but the whole idea makes me think at a fanfic I read a while ago (partly I gave up because of the lenght) ( http://www.fanfiction.net/s/5178251/1/30_Minutes_That_Changed_Everything )
Now back on your questions. First what is it like.
Imagine a three less world. the few men remaining travel, in small groups or tribes, sibbes if you like, between the ruined cities. The world has been destroyed by major war. But now the difference: not all countries are destroyed. No the big ones, because they are defeated, nor in the richer part of the world because it was way too easy to get involved. Only a few in Africa and Asia. Although they have changed, some countries have gained ground others have lost ground due to infertile ground but most countries are newly emerged: small half barabarian little civilazations around ferile grounds or oilwells or other important resourses. The oil is sold by the owners to the countries who don't have any so they can keep their tanks running (some countries are able to build armoured vehiculs and sell them to others) The Weapon manufaturers and oil well owners rule the world.
Most of the strategic resourse countries are forced to join the Union of Free Nations which means that they are controlled (suppressed) by one king, one king to rule them all ; ) This means that the smaller nations focus on producing as much of food to ensure theirs and the Union Of Free Nations (UFN) needs, in order to stay independant they also don't look around for any resourses, or if they dare to do so they dig up so little no one's interested.
Only on small islands in the oceans theire are free nations who joined together in a Free Bond of Islands (FBI) To keep what they have.
The little nations and tribes outside these two groups fight each other all the time, the strongest tribes have armourd vehiculs, the weakest don't. During the time people are killeing each other, nature tries to re build itself, but short sighted humans start to use any firtile ground left until it's useless. Future isn't kind.
What caused this future, it could be that an astroid hit the earth killing billions of people at once and destroying cities forests etc... Also it would create so much dust that the world is darker and the firtile lands are where it is lighter. So no blue sky.
Another option is a global war used in the beginning of the shot, but is overused.
The last option I think is reasonnable is a combination of the two previous ones: Astroid hits the world and leaves only a few places with sunshine, and wars rage until the balance I explained above is reached.

My view your can be different, but I hope this helps.