Growing up in the good ole US of A I guess I take for granted the fact that I can speak English passably well, and even put sentences together in a respectable fashion. One might almost forget that we speak the most convoluted language invented in the last few centuries. This fact becomes all too clear when you start trying to teach the next generation how to actually USE our language. My two year old son is currently learning how to speak. He actually does a good job of it too, he is just not speaking English quite yet. I was trying to teach him along with one of his little videos the other day and the word of the moment was "flower". I grew up on the fonix method and so I started out with "F", "Low", "Er". You may see the problem right away. How does one pronouce "low"? So I started looking at the rules that cause the "O" to be pronounced differently. It cannot be the "er" at the end, because "lower" is not pronounced the same as "flower". Maybe it is the "f" at the front, because "glower" has the same pronunciationg. But... "blower" doesn't, so, no, that cannot be the rule. Then I realized that you also have "flour", a word that sounds EXACLTY the same but is spelled differently and has a completely different meaning. That is about the time I decided that he will either get it or he won't, because my head hurts just trying to think about it all.
I wonder how many people have dropped an MMO for the exact same reasons. I see threads all the time from people wanting "harder" games that don't "hold your hand", and I think to myself, "Who are these crazy, masochistic people?" Who wants to play a game where the rules, goals, and steps to accomplish those goals are not clearly stated and easy to follow? Have you ever played a game of Basketball where some guy just gets to randomly create rules and you don't learn about them until you break them and get punished? Come to think of it... there was this fun Card game I played once where the point of the game was to figure out the rules by watching how penalties and rewards were doled out... but it was an odd game.
I think too many times people confuse making a game more mature or perhaps more intelligent with just making it more Obtuse. Just because it is hard to know what to do when I step into your gameworld does NOT mean you have created a better gameworld. I need to see a familiar set of patterns, and I need those patterns to follow a standard rule. Some people think that games that follow a standard UI scheme or Quest scheme are just lazy... I think they are smart. All RTS games follow the same grouping scheme, double click a unit type and all the units of that type get selected. Then you can assign them to a "group" and perhaps even group certain groups together. Within 5 minutes of beginning almost ANY RTS I can have a good grasp of the basic commands because they all follow a common ruleset. That is not a bad thing. The interesting part of your game should NOT be the rules that allow me to interact with it. Content, Goal structure, Competition... those should be the fun parts.
I almost always come back to Chess as one of the most perfect games of all times. It is the epitome of "easy to learn, difficult to master". The brilliance, the complexity of chess is not in the ruleset or the complex user interface, but in the fact that there are multiple ways to play the game and the way the game plays is very dependent on WHO you are playing against. That is what should make my MMO challenging too. Offer me a host of possible solutions that are all clearly available given the rules, and then give me and endless number of opponents who use all kinds of different strategies. Then you can be confident that I will spend countless hours learning all of the complexity that will naturally arise from your game.