A while back I wrote a piece called Rules Rule, but Shouldn't Rule Everything. The upshot of the article was that you have to have rules for things to work, but you also have to understand the rules to be able to bend and break them when needed.
Rules are really important in gamified systems, they collapse without them. Some rules are explicit and set by the system. These are the ones that you can't break without hacking or breaking the system. Others are implicit or implied. These are the ones where trouble can sneak in.
When you play a game, you are entering into it knowing it is a game and that the point of it is to have fun. You agree to enter what is know as the Magic Circle. This is a kind of informal barrier between the real world and the virtual one that they are now inhabiting. It is an understanding that they are not the same and that you have voluntarily chosen to be immersed within the rules of the virtual world.
Within gamification, it is sometimes argued that there is no magic circle as it is not a real game. There is also the issue that some gamified systems are not totally voluntary - so how can the user agree to play by rules they have not chosen to accept being imposed upon them.
Assuming for a moment that everyone who is in the gamified system is there of their own volition, there are still some who will not be as interested in the magic circle as others. If you look at my user type diagram, you will see that in the centre there is a circle. The bottom half is solid, the top is broken. This represents how those users will view the magic circle you have created.
Socialisers, Players and Achievers will respect it for the most part. They need the structure the system gives them to be able to get what they want from it. Philanthropists, Free Spirits and especially Disruptors are not as interested. Free Spirits will be constantly looking to push against any boundaries between the virtual world and the world they want to explore or create, whilst disruptors are likely to want to change it entirely. Philanthropists will view the magic circle as great if it helps them to help others and a hindrance if it gets in the way!
So whether you feel the magic circle exists or not, make sure that any rules you want to have to make the system fair are visible or hard coded and unbreakable! A good example that catches many people out, is preventing repeated simple activities from giving people infinite points! (Like refreshing a page a million times....).
Another huge lesson that relates to this, people are not all the same. They will not "play" the game the way you expect them to. When Markus Persson made Minecraft, I very much doubt he expected people to create working computers or full scale models of Game of Thrones. He probably had not predicted that people would create versions of Pac-Man or parts of the Star Wars Movies. All of these emergent activities were born from the imagination of those who didn't want to be restricted by general conventions and ideas that others had.
This is the kind of mentality that you will face in any system that has rules and will be amplified where the system has rules that try to restrict or change certain behaviours. If there is a loophole in your rules, someone will find it.
You can't assume that everyone will agree to the same social norms as you. Be prepared for this and make sure that the system you create is flexible enough to handle the unexpected!