Rules Lite or Rules Heavy

What's in the Toolbox?
One way to look at any role-playing game rule book is as a toolbox of game components. I mostly play fantasy RPGs and the games all have a combat system, a character generation system, and a magic system. Some have a bestiary, some a skill system, some have a setting described, some don't. Some rule books are long, some short, but because of my own philosophy I approach them all as guidelines rather than strict "rules of play" to be followed slavishly. So as far as I am concerned, they are all toolboxes for me to pick and choose from so I can (try to) create the kind of game I want to referee. I frequently borrow an idea from one game to use in another, mix and match, one might say. I assume this is because my first RPG was White Box - a system that rather requires this do-it-yourself approach, but it is also a preference.
As a result, I generally run (as referee) a game that openly states "this is your game, change whatever you wish..." Frankly, I do that whether I am given permission or not and players are told this up front. As referee I try to be fair and mostly consistent and I often discuss how we will handle something in-game and use player input. In honesty, there are some rulings I have made just on my knowledge and preferences - how I see the situation in-game effects the odds of success or failure. I like the freedom to experiment, to mix it up and keep it fresh - to use my imagination. I like outcomes that make sense to me and that I can narrate once the dice are rolled. I like to reward clever role-play and innovative problem solving with a flat-out "yes, that works! No dice roll necessary."
I am of course aware that unpredictability can be bad for players. It can make it difficult for them to judge risk. Rulings can be disagreed with and seem arbitrary. I have played with a few referees that didn't seem fair and managed their game table in a way that didn't seem fun to me. The referee can make a big difference in the game experience whether the rules are short and loose or long and tight.
Some rules authors seem to try and make their game proof against poor referee skill. The rules take away as many opportunities for the referee to use their poor judgement as possible, but that term "as possible" is important. The referee still chooses the setting, role-plays the NPCs, controls pacing, acts as the eyes, ears, noses, etc. of the PCs by describing what they see, hear and smell and therefore can still "muck-up the job". It is not possible to write rules that will prevent a referee from doing a poor job.
What happens when rules become too tight, when they take too many choices away from the referee is to stifle creativity. To borrow a metaphor from art, oil paints are pretty open ended. The "artist" can create on canvas, mixing pigments and varying brush strokes, almost any image imaginable. Some "paintings" are artistic, some are even "masterpieces" and a lot are frankly bad and uninspiring. In contrast working with Colorforms (a child's set of vinyl shapes that adhere to a printed board) anyone can compose a picture that is recognizable (cartoon characters, TV show actors, etc), but the medium limits the creativity. "The Last Supper" done in Colorforms (or paint-by-number) is severely lacking compared to the original done with oils. In a similar vein, rules that are flexible and loose (even vague in places) allow greater freedom to interpret and create than rules that try to be "complete" and define too much.
Which type of game is best (most fun) depends on personal preferences. Is there room in our hobby for both approaches - absolutely. There are times I personally like to delve into a thick rule book with lots of detail and try to master it all. That can be its own brand of fun. Mostly I prefer games (with fewer rules) that promote a maximum amount of freedom for the referee (and players) because I enjoy seeing what people come up with. Part of my entertainment is being surprised by another's creativity. There are lots of ways to enjoy the hobby and I am not one to label anyone's game "bad-wrong fun". (I have some fond memories of imaginative play with Colorforms!)