As has been said oversupply and a fall in demand are the main issues. China's rate of growth has slowed and the EU is still on the brink of recession, in the medium term a lower oil price will help kick start economies round the world and so increase the demand for oil. Oil supplies have recently come back on line from Libya and we are just starting to see the impact of shale oil and gas from the USA.
The high oil price over the last few years has led to the development of unconventional oil and gas in the USA, traditionally these were too expensive to produce and were therefore not developed. They are still expensive and require continued investment to drill new wells, as each well has a very short life before it is depleted. A lot of the fields will be uneconomic below $75/ bbl; over the next 6 months production from the US will tail off, this will be temporary, as they can easily start drilling and fracing again if the oil price makes it economic. A lot of people in the industry expect the oil price to stay around $60/bbl for the next six months and then settle around 80$/bbl.
Lots of jobs will be lost in the NE of Scotland over the next few months, with the oil price already below the break even cost for a lot of the older platforms. And new projects will be cancelled or put on hold.
A low oil price my be good for world economies but could be a disaster for the environment - there will be less incentive to develop alternative energies and more CO2 will be released into the atmosphere.
I think all the talk of punishing Russia or Iran are just conspiracy theories, it is just the market adjusting to changes. Personally I think that Russia becoming unstable is highly dangerous and the Russian people are likely to back Putin against the world if they are backed into a corner.
All the above could change if IS continue to spread across the Middle East and into North Africa.
happy new year ;o)