The rejection of the pipeline is good news for many reasons.
By rejecting pipelines, the jurisdictions around Alberta can slow the development of the oil sands and reduce the total quantity of fossil fuels that will be burned. These pipelines are also a major investment in an inappropriate technology. Canada needs to be working on developing a decarbonized economy, not encouraging unlimited growth in the unsustainable business of extracting fuels from the oil sands.
President Obama will probably lose a few votes over this decision, particularly from people who think oil is still the future of energy and who do not care about climate change. At the same time, I am sure he will gain some votes too for finally doing the right thing on this. The choice offered to us by the oil sands is to either profit today in a way that harms future generations or to leave the oil in the ground and invest in safer sources of energy.