What Makes a Great Sandbox Adventure Great?

For going on six months now I have been doing an on-again/off-again read/reread of favorite or suggested sandbox products. Unbidden the same driving question keeps popping into my broken brain: what particular set of things makes this product so damn good? (What makes it shitty or mediocre being an interesting and useful question for another time.)

I have something like 30 unfinished blog posts beating, beating in tell-tale-heart urgency from my draft box and some of them are various attempts to move those margin notes into a blog series. Reckon it's time to whip those recalcitrant posts into publishable shape.

Before I kick off the series though let me set the frame. Imagine the standard disclaimer here these elements are what make a particular sandbox pop me for me, your own mileage may vary blah blah blah. You will note that how I frame the criteria precludes a number of much beloved sandboxes and hex-crawls.

Lines had to be drawn.

That said I am curious to hear if you the reader were making this list what would you include? What particular thing inspires you or helps you run it? And what products would you through on your High Fidelity Top Five list?

What constitutes an exemplary sandbox adventure:

• Main frame is a large, but bounded wilderness or outdoors environment.
• Open ended and allows (may even explicitly plan) for different outcomes and play (though most often having loose overall goals or player motivations wired in).
• Combines the adventure-site exploration with another deep axis (political maneuvering, interesting NPC goals, timelines or whatever) to make it multi-dimensional.
• Site description is in my utterly subjective “sweet spot” (between terse hex crawl and baroque setting books).
• Has distinctive color and texture.
• Often has something that enhances or bends the system it is designed for (say mechanics for running particular natural obstacles or mythical wildernesses that bend standard rules).
• Has a map that is either aesthetically inspiring or has interesting play choices built in. 

Sandbox adventures to be examined (ranked loosely by personal inspiration):

• Griffin Mountain (Runequest)
• Twilight 2000 adventures
• Heart of the Sunken Lands (Midkemia) 
• Vault of the Drow (AD&D)
• John Stater's Land of Nod/Hex Crawl Chronicles
• Pitzburke (Gamma World 2e)

Almost rans (ballpark goodish, that may or may not be thrown in):

• Leviathan, Prison Planet, Tarsus and Beltstrike (Classic Traveller)
• Isle of Dread (B/X)
• Night's Dark Terror (BECMI)
• Qelong (LoTFP)