Tai brings up a good point, it is incredibly hard to pinpoint the most "important" global environmental problem. How can one say that deforestation is more/less important than rising sea levels? It would be hard to convince anyone with strong convictions that one issue is more important to mankind than another. That being said, I think that biodiversity loss is one of the many important global environmental problems. It is among the most important because it is impossible to reverse.
The large variety in species of flora and fauna on earth has come about through billions of years of evolutionary mechanisms. Through the mechanisms of natural selection, species naturally go extinct. There have been five previous mass extinctions that have been seen in the fossil record. However, the current rate of extinction is one thousand times that of the fossil record. What this means is that we are in the middle of the sixth mass extinction. This extinction is believed to be caused by human behavior. Changing natural habitats, introducing non-native species, and over hunting have all contributed to this mass extinction. Once these species are extinct, there is no way to bring them back. The reason why this is such a big problem is there is no feasible way to reverse this extinction through technology or changes in human behavior. Once a species is gone, it's gone for good. It is possible to clean our rivers, re-plant trees, and stop emitting greenhouse gases, however unless we develop some kind of Jurassic Park technology there is no way to bring these species back. The only thing we can do is stop the bleeding, and globally change our practices. As you can see, there are no easy answers here.