The problem is not the pipeline itself. Large pipelines are standard engineering. The disruption is fairly short term.
The problem is that it would make a ready market for Canadian Oilsand oil to get to the gulf coast refineries. This drops the price of oil on a long range basis. (The Athabasca tar sands have more oil in them than ALL of the known recoverable reserves in the world.)
Worse: Because the oilsand oil takes so much energy to extract, the net CO2 released is about 1.5 times the CO2 from a similar amount of conventional crude.
There is some concern about the possibility of a spill. There is some justification for this fear. See the response to other oil pipeline leaks. One way around this would be to require the pipeline company to post a bond to cover a spill. They would receive the interest on the bond, but the bond would be available to finance recovery.
@THelper points out a wiki article with lists of spills in the US. http://en.wikipedia.org/wiki/List_of_pipeline_accidents_in_the_United_States
Propane and natural gas however have little immediate environmental impact. (NG is a greenhouse gas.) Many of the spills listed don't say how much spilled. If the safety systems work properly, the line should come to a halt with only the contents of a mile or two of pipe spilled. A million gallons is about 3 acre feet. Big nuisance. Expensive cleanup. But still a local problem. Pipeline companies need to be held accountable to a greater extent than they are now, so that spills are more expensive than running safe lines.
Overall a large pipeline has far less impact than transporting that same oil by rail or truck.