The most immediate characteristic is as a source of energy. However once the price of oil gets above a certain level then we stop using oil just for energy. The first step in this process was when people stopped using oil for electric power generation, which happened 40 years ago. The next step, which is happening rapidly in North America, is that people stop using heating oil to heat their houses in winter. We are not running out of energy. Indeed it seems certain that we will get Nuclear Power working and producing cheap electricity well before there is any shortage of coal or natural gas. Claims that we might run out of Uranium are silly: it is precisely because there is so much cheap Uranium that we can't get more modern reactors up and running that would use much more of the nuclear fuel and leave much less residue.
The 2nd characteristic of oil is as an energy carrier. Liquid hydrocarbons are the ideal fuel for transport. And we have an enormous infrastructure using that. But the oil price has already disengaged from the price of energy. Natural gas is cheaper, electricity is cheaper if it doesn't come from silly renewable sources.
So Peak Oil folk talk about oil's declining EROEI: Energy Return on Energy Invested. We can see that EROEI is declining markedly. But this is now irrelevant. Oil is no longer being used mainly as an energy source. It is being used mainly as an energy transporter. The Peak Oil folk are quick to rubbish "The Hydrogen Economy". Hydrogen is not an energy source at all, and it is easy to see that it is a very silly energy transporter. Time to wake up and rethink oil as energy transport, and stop worrying about its EROEI.
Once we stop worrying about its EROEI, we can see that it doesn't matter if Canadian tar sands or very heavy oil requires a lot of energy to extract. We could even make oil out of thin air, using algae for example. The EROEI no longer matters.
This is not to say that we aren't going to have a nasty decade or two. At the end the successful countries will be the ones that make Nuclear Power work. Certainly India and China agree on that point.