The problem with releasing the full dataset is two- or three-fold:
First, the large download size. No matter how you look at it, it
will be an initial problem.
Second, the full dataset containing EVERY land area on the planet Earth would be more less redundant for the purposes of a demo ... There's just too much too look at. Yet islands would provide hours (if not days and weeks) of exploration, and help alleviate the first problem. I'd be more or less content with a "lesser world," just to experience what the developers have been experiencing and teasing us with
Third, the full dataset IS the biggest feature, and some things should be held back for teaser reasons. It could help generate more enticement for developers, just experiencing a little of the dataset and leave them wanting more.
As for the "qualified forum members" thing, this thing has been used in the past with almost every game development alpha release. Some members are selected, usually based
first on system requirements, and secondly by those who've been around awhile and actively engaging the development (as well as be willing to truly submit bug reports etc). This is not much different, if at all.
It'll be disappointing not to visit certain areas, but this is just a
demo, a demonstration of what WILL be possible when the engine comes into maturity.