I think I heard once something called the rule of seven. Basically it means that a person can't really have more than seven actions on his mind at the same time.
This means, that what actually can be considered complex and boring is not so much how hard something is, but how many actions it requires you to master at the same time.
An example would be games like stepmania, that can be really fucking hard but they usually don't have too many bottons or actions to make, so they are simple in concept, hard in mastering.
The rule of seven basically tells you about seven actions you can do at the same time, in a global sense, and have to control all of them to play.
Using a game everyone knows like Dark Souls, for example we would have the camera control, the strong and soft attacks, evading, blocking, sprint and items. Those are seven actions you basically use to play though the game and that someone can master them at the same time.
Giving more actions make it more complex, and less actions less complex. In the case of Dark Souls we could also say there's also riposte, but actually is an intelligent game because depending of your character you would go for a more evading/rolling approach, or a blocking, riposting approach, or a magic and keeping your distance approach, etc... so at the end it is still seven actions you have to master, and trying a more broad approach of actions it's just for the players that wish to do so, but doesn't force you to do it.
Of course, you can change this seven actions depending of the context. For example when you come from a dungeon and go to the city, you change your mental switch and start thinking in economy terms, looking for new weapons, armors, information, etc... Here you should apply the seven actions rules too. Think of the seven actions as the stuff the player should have in mind during the present moment he's playing. So when he goes to the city he changes his battle seven actions for the social seven actions for example.
The seven actions are something you can see everywhere, and it basically tells you that when something asks you for more than seven actions people usually gets overwhelmed, and need to actually train themselves to dominate them. So at the end it's a good rule to try to keep your game below seven actions at the same time, so all the player can have in mind at the same time are those seven actions, nothing more and nothing else. Then divide those seven actions (or less), teach them individually, and make it for the player to learn them one by one, then combining them together later to have this sense of mastery and fun.