Themepark isn't the reason for modern MMO successes, its accessibility.
Older sandbox games were simply too complicated for the average gamer. Hell, my first month in SWG was a nightmare! Dumped on a planet with no weapons, no tutorial and no clue! Its was dire! Luckily, I had a friend to teach me and so I stuck with it, learned the game and it has been my favourite mmo to date.
Simply being a themepark is not enough to hook players. The best MMO (yet to be built) will have a combination of sandbox and themepark elements. Themepark elements are the best way to guide people through a game, for example classes, levels and linear questing. Sandbox elements are the best way to keep people playing long term, for example player cities, player made pvp wars, strong crafter-based economy, dynamic quests, random high level gear drops, dynamic / random open world raids etc.
A good example of this in the single-player world is the Elder Scrolls Series. These games include the best bits of themepark design (linear quest lines and dungeons) with the best bits of sandbox design (open world exploration, complete character freedom, strong crafting, choice of quests). As a consequence, the Eldar Scrolls are some of the most popular RPG games ever made.
I agree 100% with the bold text here, I to played SWG and was very confused when I first landed, the community that was there when I started pre cu, was so helpful someone showed me the ropes, this game does not confuse you which is great but some of the sandbox elements would keep people amused while new content was made.
I have to say though one of the biggest things I find is actually not to do with the game, when things go wrong the silence from Bioware employees is so frustrating. How many people left who had no actual complaints regarding the game but couldn't for example log in or play due to poor patches? More responses from customer services that are not cut and paste so you actually feel like someone has read your text would go a long way to alleviating the hate on these boards.