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When I think of sandbox's I think of alot of systems in place that encourage and reward mundane things. Like Crafting, Building and Socializing. Those things are important but only to a degree because the most enjoyable experience of any MMO has always been new adventures and new places to see followed by a wonderful progression system.
In Neverwinter, the Devs have implemented what I think is the #1 element that makes an MMO fun...."NEW CONTENT". Neverwinter utilyzes the Foundry system to allow custom content to be created that concentrates on Combat, Adventure, Exploration and Progression....Arguably the biggest and most important things to do in an MMO, and arguably the things that make a Themepark more fun in the short term compared to a sandbox.
What happens in 99% of all MMO's is the game starts off fun but eventually you exhaust most of the developer created content and you get bored (Read GW2) and there eventually comes a time when content can't keep up with the instatiable appetite of the players resulting in player loss. Neverwinter corrects this because during the long periods of development time needed to produce good content can now be filled with player made content. So in essence, NWO becomes the best sandbox game because it offers plenty of long term content to appease the playerbase while still concetrating on the 3 elements that make Themeparks more fun and more enjoyable then their sandbox cousins.
This is my opinion only, and is the great reason why I thiink UGC is cure all for all themepark woes.
Sandbox means open world, non-linear gaming PERIOD!
Subscription Gaming, especially MMO gaming is a Cash grab bigger then the most P2W cash shop!
Bring Back Exploration and lengthy progression times. RPG's have always been about the Journey not the destination!!!