Okay, here's a question. You seem to be assuming that a meaningful choice has to have a unique right answer. But the hallmark of an interesting choice is that it doesn't have one.A decision, as a response to a choice, is something that helps the player reach his goals. For example, these goals can either be doing maximum dps, or having a cool looking character – or both. Different goals induce different “unique right answers”.
We can use the Extra Credits terminology: calculations, incomplete information problems and incomparables.
Calculations can be thought through before you make a decision, incomplete information problems could be thought through before you make a decision, but you miss information and usually have to make an educated guess. Incomparables make you ask yourself “What do I actually want? Do I want the apples or the oranges? Do I want to look cool or do more dps? How much less dps is a good look worth – and vice versa?”
Calculations and incomplete information problems have a unique right answer, because they have a clearly defined goal. Incomparables don't have a clearly defined goal. They require you to make up your mind about what you actually want – something humans generally enjoy only in small doses in my experience! Should you make up your mind, an incomparable becomes a calculation or incomplete information problem, but many players never ultimately make up their mind. Consequently, the incomparable stays an incomparable forever and only generates transitory calculations or incomplete information problems.
Incomparables tend to produce less satisfaction compared to choices with clearly defined goals, because the player never really reaches his goal: the correct decision. Incomparables often are frustrating as the player really wants to have the apples and the oranges. Most players want to look cool and do maximum dps. They feel like they shouldn't have to choose. The good thing about incomparables is that the act of choosing never ends. It is therefore cheap 'content', a cheap way to keep the player's mind busy.
For example, the choice of deck build in Magic the Gathering is interesting. It's not a "flavor" choice (because some decks are more effective than others), but it can't be solved using a spreadsheet (because making a good deck isn't a matter of maximizing a single variable like DPS). You could same the same thing about the choice of move in chess, or block placement in Tetris.MtG is an incomplete information problem. If you knew your opponent's deck and how he reacts to your actions you could easily find a unique right deck that maximizes the chances of a win. This, by the way, is the single variable you try to maximize.
The reason this works in MtG and many other multiplayer games is that players accept the fact that the opponent behaves unpredictably and sometimes irrationally or even arbitrarily. If a boss in a raid behaved this way, most players would consider this frustrating. They wouldn't feel like they should have to fight such an opponent. It would seem unfair to them. A good example for how essential the players' expectations are!
So here's the question: Why can't MMOs present choices without a unique right answer? Or do they already?They easily can and sometimes do. But usually these choices just aren't fun. Humans like to work towards clearly defined goals, like how to look coolest, or how to maximize the dps. But we really don't like to make up our minds. A lot of people even actively seek to stop themselves from thinking about this. That's one reason we marry and get kids and try to be successful in long-term jobs. Trying to achieve something - anything - is generally more fun, than trying to find out what we actually want.
One could add lots of choices to a MMO that don't have unique right answers. For example, one could force players to choose between leveling fast or experiencing the main story of the game while one-hitting all mobs. WoW does this right now. It is an incomparable. It doesn't have one right answer. It depends on what you want. You need to make up your mind! Do you like this choice?
Sandbox games like Eve Online are a lot about making up your mind. Do you want to become a rich trader or a pirate hunter? Do you want to do PvP in null-sec or stick to high-sec? Even players who really want to like these games (like myself) suggest that new players just select one (any) goal and stick to it. Pondering about what you want is not a lot of fun. Switching around can be fatal for your fun.
One of the few incomparables I consider arguably fun is selecting your character in games like Neverwinter Nights. Do I want to play a lawful good paladin or rather a chaotic neutral rogue? Should my wizard be balanced or super-intelligent? This can be fun for quite some time. I tend to spend hours in front of character creators. But I absolutely know that, eventually, I need to decide and stick to my decision to have fun.
So, to reply to your comment: all choices, except for incomparables, have "unique right answers" xor are meaningless. For a choice to be "interesting", it can absolutely have just one right answer. And while incomparables (which never have one right answer) can be interesting, they are also very difficult to get right as a designer.
The real problem of "right answers" is that the act of choosing eventually ends. And unless it is repeated, its potential to create fun for the player is thus gone forever. Luckily, most choices in games can be repeated, often under different circumstances.