From the World Heritage inscription:
Located in the north-west of Washington State, Olympic National Park is renowned for the diversity of its ecosystems. Glacier-clad peaks interspersed with extensive alpine meadows are surrounded by an extensive old growth forest, among which is the best example of intact and protected temperate rainforest in the Pacific Northwest. Eleven major river systems drain the Olympic mountains, offering some of the best habitat for anadromous fish species in the country. The park also includes 100 km of wilderness coastline, the longest undeveloped coast in the contiguous United States, and is rich in native and endemic animal and plant species, including critical populations of the endangered northern spotted owl, marbled murrelet and bull trout.
I think this is one of the more underrated National Parks in the US. It is a rainforest, albeit not the tropical kind people usually think of. The mosses and the pine trees create a surreal effect when you are walking through them. It is also the location of Forks, WA which as we all know is the home of sexy vampires.