I'm not really into this game's theme so I haven't checked it out, but since I follow the industry and my MMO roots are in sandboxes, I understand that it's one of the best sandboxes out there. I remember early reviews were making comparisons to Ultima Online and EVE (mechanically, of course, not stylistically).
Some stuff about sandboxes though, for the uninitiated.
Unlike "theme park MMOs" like City of Heroes, sandbox MMOs often do not lead you by the hand between contacts, story arcs/quests, and raids, especially outside of starter zones. They might not have any of those things. Sandbox MMOs can be daunting for the inexperienced, or outright confusing, because it can feel like there's "nothing to do" in a world that might be filled with things to do. You usually have to set your own goals or join a guild and ask what there is to do. Those goals might be common and expected (chop trees and hunt wildlife, build a house, then get better and kill a dragon for its gold, build a castle), or less common (explore every shrine and every level of every dungeon in the game, without using any teleportation powers). You probably won't get badges for many of the objectives you achieve, because many of those objectives will be ones you made up for yourself.
These games often require resources for crafting, which are not just won from monsters but frequently also obtained from environments within the game using special skills; and rarer materials might require specialization, putting a premium on veteran crafters services to players of other professions.
In a theme park MMO you generally can't lose items you've won, and are locked into progression paths such as classes (or archetypes). In a sandbox MMO you may be able to lose your gold, weapons or other items if a monster (or player) kills you or steals from you, or you may even suffer stat loss if killed or skill atrophy if skills go unused. However, some sandboxes soften the blow by allowing players to insure certain items against the possibility of losing them, either gaining the item back or a portion of its value upon resurrection. The possibility of stat loss varies between games, but compensating for this unfair fact of virtual life, most sandbox MMOs feature classless, level-less progression: you gain skills by using them (or purchasing training), and therefore a single character could master one career, change careers throughout its existence, or hybridize.
Sandbox MMOs are often designed around a challenging philosophy of risk versus reward. You can log in and set small goals, but significant success will usually require longer-term objectives. In theme park MMOs, resource-gathering and crafting are often treated like mini-games. In a sandbox MMO, you may spend days and weeks at a time simply extracting materials from the environment while fending off (or running away from) monsters.
Sandboxes can be extremely rewarding or extremely punishing and disappointing. You really have to experience them to know what that means. Working for a year to buy or otherwise acquire a rare and powerful item feels good. Losing it in battle, along with an entire set of expensive armor, a bag of gold, and a sentimental item you should have banked, can be upsetting in a way that you never expected from a video game. The risk of the latter and the associated rush of the former is one reason these games sell; but many other players simply log in to hang around safe areas, chat, craft, and decorate their homes.
