Tobold has recently posted an interesting article about "sandbox" games vs "theme-park" ones. For those who aren't familiar with the terms, "sandbox" MMOs are those where players are given a lot of freedom to make their own fun, whereas "theme-park" games have an emphasis on developer-provided content, but offer fewer options.
Of course, sandbox and theme-park are not binary states; they're poles that any game can sit between. All MMOs have elements of sandbox and elements of theme-park, but some have more of one and some have more of another.
What's interesting from a design point of view is how that choice affects the content. By making a game more theme-parky, you are guiding players in specific directions, but that means you can focus your developer effort in those areas, to produce a high quality experience there.
More open games suffer from the problem that developer effort is, by necessity, less focussed, so the average experience is worse, even if you have more choice. To some extent they attempt to fill this gap with "player generated content". For me, this is one of the most interesting areas of MMO design. At its best it can lead to epic battles between different groups of players as in EVE. At its worst, it can lead to unrestricted ganking of players who are minding their own business, as in ... errr... EVE.
Before you ask - yes, I do know that both the above links lead to descriptions of the same event. That's the nature of player generated content - it can be both good or bad, depending on your perspective.
Is there a sweet spot between the two, where the limitless content offered by a sandbox play style can be blended with high quality developer-provided content? I'd certainly enjoy WOW more if the economy were more open, with buy orders available on the AH and real options available for high-end raiders to profit by selling-on their BoE rather than BoP items. These are fairly small changes, but they could open up new ways of playing: for example, gearing up by making money selling services & using that money to buy high-end gear you can then choose to raid in. On the other hand, I don't want to get ganked every time I go to the shops.
Choice is good, but when that choice includes the right to deprive others of their preferred play style, that's where I draw the line. There's a fascinating reflection on the issues associated with this kind of "choice theft" in China Miéville's Bas-Lag series, where it's the basis of the Garuda legal system. Excellent books, which I suggest you go check out- they go way beyond the regular fantasy Orcs vs Elves cliche. Now there's an idea for an MMO...