So people have been referring to this game as "themepark MMO" but recently I've been questioning if that term really fits. Themeparks are marked by developer driven gameplay. They set you on a path and you ride the rides. To some extent, that seems to be true for this game's initial PvE experience. There are leveled zones that the player progresses through in sequence, completing content and then moving past it.
On the other hand, it seems to differ from traditional themeparks in important ways. To begin with, its been said that there are no obvious signs (other than a small load) that you are moving between zones. Things like the big blue energy gates in GW2 make it very obvious that zones are separate from each other. Without those, the world is going to feel much more integrated and "sandboxy" if you will. Just the perception that the world is continuous is an important change in my mind.
More to the point, player actions are not placed "on rails" by the developer. To continue in the main story, you will have to complete certain missions before you can get to later ones. But that was true of Oblivion and Skyrim as well (though not Morrowind.) Much of the rest of the content, such as side quests, dungeons, and exploration, is optional. Players choose where to go and in what order they wish to complete the content. Exploration is entirely up to you, but you are rewarded for scrutiny with skyshards and Mundus stones.
The question for me is, are leveled zones really what differentiates a sandbox from a themepark? If that's true then Morrowind is a themepark RPG. In that game, enemies did not scale to your level. Their stats were fixed. And some areas had more difficult enemies and content than other areas. Woe be to the level 1 Morrowind player who ventures into Red Mountain. Yet, Morrowind is in many ways the most "sandboxy" of the Elder Scrolls games (as mentioned earlier you can complete the main quest without doing almost any of the prerequisites.)
On top of all that, the things that can be claimed to be "themepark elements" only really apply to 1/4th of the content. Your initial PvE play through in your home faction has leveled zones, your PvE in Cyrodiil, 50+, and 50++ do not. Cyrodiil scales everyone to level 50, is a contiguous zone, and does not lock any content based on player level (after you're allowed to enter at level 10.) It is, in every sense of the word, a sandbox. That's not even to mention PvP (which is an inherently player driven activity) or destructible environments (a staple of sandbox games.) Many of the same things can be said about 50+ and 50++. Once you enter a second Alliance zone, all content is scaled to level 50. That means there is literally nothing stopping you from completing all content in that Alliance's territory in whatever order you choose.
For what its worth, I'm apparently not the only person who thinks this way. Several recent gameplay impression articles have described the experience as "open world" (such as http://gamerpvssyr.com/2013/08/05/preview-the-elder-scrolls-online-from-quakecon-2013/
I think ESO is best viewed as a hybrid between a themepark and a sandbox. And I think that's great. By taking the more focused and meaningful story of a themepark and combining it with the player freedom and world immersion of a sandbox, ESO will have an edge over both traditional themepark MMOs and upcoming sandbox titles like EverQuest Next, Black Desert, and ArcheAge.
Do you agree?