It was the first attempt at a more sandboxy style of island.
There was lots of stuff to discover and collecting things was still new and exiting.
It looked and sounded really nice.
It was easy to travese and rarely frustrating.
It was part of a really great expansion with an engaging story and good gameplay, so people weren't quite as checked out of the expansions story/gameplay as much as with other .2 or .3 islands.
Not saying it was the best island (that's probably the mechagnone one), but being first and being part of a good expansion really helps being remebered fondly.