The uniqueness of Galapagos is largely due to the remoteness of the Archipelago which has never been connected to the lands of the South American continent.
This has left the animals, plants, bird life, reptiles and marine life who have either migrated or arrived by sea to develop somewhat differently to their mainland brothers and sisters, for example the various types of reptiles inhabiting the islands are all exclusive to the islands while 25% of the nearby shore fish and over 50% of all plants are also unique only to the islands and found nowhere else.
Another interesting facet of the Galapagos is the sheer fearlessness of the wildlife, for instance you can easily swim eye to eye with sea lions, penguins and other marine life without them blinking an eyelid or go follow a giant tortoise as it makes it way pedantically through the lands, in fact the islands were completely uninhabited by man until 1535 and for the wildlife living there man was just another species washed up upon the shore and nothing to fear. It is also a paradise for snorkeling and you can view several multicolored glistening fish and invertebrates including rays, turtles and sharks.
For natural historians or those interested in geological and wildlife development. The islands are a paradise and a step back into time where little has changed for hundreds of thousands of years, they are not however for the sun worshiper or seeker of beeches because the scenery is more volcanic with barren wilderness and more serene in nature and is not for loud party types or regular mainland tourists.