Generally, themepark MMO's are the ones that have developers set out predefined paths for the players to follow, allowing for a consistent experience with a high level of quality. In these games, you'll hit very little highs (and lows!), which generally falls well within mainstream gaming. Even if you argue that some of these MMO's have a big world with generally a lot of freedom, there's usually just a few (if that) paths to follow. Even with group gameplay, your role is pretty much fixed (a good example is the tank/heal/dps element in almost all of these games). And that's fine, people playing these games have expectations, easily met by the developers.
Sandbox games are pretty much the opposite. There might be themepark elements (like the tutorials for new players, or the assignment-relation mechanics), but these games allow for any path or gameplay style, within the game's mechanics.
After all said and done, in my opinion sandbox games do one thing very well; the highs are extremely high. Then it's up to the players and their expectations to negate the inevitable lows as a result of this. The developers just provide the tools.
Then again, analogies are just that. They might be interesting. but don't get stuck on them.
