The rules in an RPG define what players can do, and can be strict or loose, depending on the system and the group.
A group that adheres well to rules tends to enjoy the structure they provide, the predictability they lend, and the power they give the players. They may define what you can do with hard-as-steel gates, but they also give you the key to those gates. Some RPGs require the players to do a good deal of system processing. Many systems give the GM one set of rules (typically limited antagonist abilities, plus campaign and story design rules) and the players another (special magic or unique combat and skill abilities). I've been running 4e D&D and Night's Black Agents lately, and both involve a good deal of "off-loading" system mastery onto the players. 3rd edition D&D and Pathfinder do this as well; as do the World of Darkness games. A major benefit of these games is that they can give the players a very mechanical, predictable, world; provide those characters a lot of unique abilities; and not burden the GM with having to learn all the rules for those abilities. I call that "distributed processing" because these games tend to have a lot of system options that only the players need to learn how to use. For players who like it, system and system mastery are empowering and enjoyable.
A group that likes to play loose with rules either enjoys the GM-as-Storyteller dynamic, or they distribute the director/author roles more equally. Perhaps they have short attention spans or no interest in system mastery. Perhaps they play RPGs to sit around a table with friends, drink beer, and come up with wacky stories. They may prefer story games like Fiasco, or games with greater narrative control in the players' hands, such as Fate; or any number of indie games. Personally, I enjoy Unsung when I want a distributed-authorship game. For players who hate system mastery, a mechanical, predictable world that makes them do mental labor they dislike to navigate it is a stahlhartes Gehäuse. They will not engage in system mastery, and when placed in a game that requires distributed processing, they will become bored or frustrated, because their dislike of the system will keep them from engaging with it, which will prevent them from effectively interacting with the game world. I call these "system-avoidant" players, which is not a derogatory term. They avoid games that put more system in between their imagination and their ability to exert their ideas in the shared imagination space. They also tend to dislike the "game" aspect of RPGs, since the game aspect usually gets mediated by rules.
The takeaway here:
If you're running games for a group that doesn't want to take on system mastery, choose a system with fewer player options. Otherwise, the players won't learn their character's abilities, and you will be forced to run both your side of the screen and theirs. They will quickly become frustrated with the situation, and so will you.
If you are running a game for a group that contains some players that don't want to take on system mastery, and some that do, you have a few choices:
1) Run games in systems with fewer player options that depend on system mastery. If the group is at least 40% system-avoidant players, you may need to consider avoiding games that use distributed processing, or at least avoiding D&D. This probably includes the 5th edition coming out later this year, despite its efforts to significantly dial back the system complexity.
2) Guide the system-avoidant player to a character that has broad ability to interact with the world, without any special rules (this may be hard if not impossible in D&D, where magic is always very complicated, and not having magic is limiting). In Night's Black Agents, just ignore special attacks and cherries, and it should be OK. In the World of Darkness, there is usually a character option that focuses on sneaking - this tends to be an easy system to interact with, with minimal mastery required.
Finally, if you're running a LARP, system-avoidant players can enjoy both elysium style and adventure style games. They will dominate elysium style games (if they do) through politics and conspiracy, rather than systems and combat. As for adventure style games, LARP systems are rarely so complex and important that a player who ignores them is substantially hindered by doing so.