I've been playing role playing games, in one form or another, for about 26+ years (I started playing Dungeons & Dragons ™ when I was 13; I'm now 39 – do the math). Which means, I've been playing these types of games for longer than some of the members of my current gaming group have even been alive (holy shit, I'm old!). There are a couple of things that make these games fun; for me anyway. There's the social interaction, for one, but mostly it's just fun to be able to vent frustrations by portraying a character that can do things that I can't – either because I don't have the skills, or because in the real world, I'd be in jail if I did many of things my characters have done in game. There's also the fact that by participating in a role playing game, I get the feeling that I'm one among a group of authors who are collaborating to “write” an open ended story. I've always wanted to write a novel, but anytime I get an idea and start writing, I realize that I'm just rehashing typical fantasy stories, or my idea is to much like some author's work, and I really don't want to get sued, so I stop writing. Playing an RPG helps to satiate my desire to write.
The balancing act of keeping the game fun and interesting falls squarely in the lap of the Game Master (aka, Dungeon Master, Storyteller, Weaver, referee, etc.). The GM, has the job of coming up with the setting and plot of this open ended story – sometimes using published works specifically written for the game, sometimes coming up with their own (or, in the case of games I have run as GM, a combination of both). The players, portraying various characters within the setting, collaborate with the GM to unfold a dynamic story.
The challenge any GM has is to not only come up with a story arc that is compelling and draws the players in, but to also keep a balance withing the mechanics of the game. Almost every RPG uses statistics and dice to help determine outcomes. It's all well and good for me to announce that my player performs a certain action, but, as in real life, success is not a foregone conclusion, so dice are rolled and compared to the character's abilities and skills versus how difficult the task is, or versus another character's ability to out maneuver or out smart the player's character. For example, a decide that my character wants to climb a wall and he has a climbing skill of, let's say +2. I roll a twenty sided die (d20), and add my +2 climbing skill to the die roll. The GM then has to decide just how difficult this particular wall is to climb; is it smooth, or does it have spots that jut out that could be used for hand and foot holds? He/she sets a target number that the character has to meet to succeed. So, for this example, we'll say that it's not a terribly high wall, and has places to grab, so the GM sets the target number at 10. I roll a 9 on the d20, adding my +2 for my climbing skill for a total of 11. My character succeeds in climbing the wall. The trick is, the GM has keep mechanics like this balanced – and by balanced I mean both fair and challenging. Sometimes, a task is very simple, sometimes it's challenging, and sometimes it's downright heroic or epic in scope. If the GM sets the target numbers to low, the games not challenging enough. If he sets them to high, then players get upset that there's no way to succeed (player characters, for the record, tend to be better than average in certain skills by design – that's kind of the point, the players are portraying characters intended to be heroes in the story). (1)
There are good GM's, there are great GM's, and unfortunately there are also bad GM's. Some people just can't tell a good story. Others have a GM vs the Players attitude. Some give the players to much, others don't give the players enough. As I said, it's a balancing act, and the truth is, not everyone who plays RPGs is capable of being a GM. And that's all I have to say about that.
(1) this example happens to be from the d20 System – there are systems that utilize 10-sided dice or 6-sided dice. My example in no way is intended to, necessarily, advocate that the d20 system is any better or worse... blah, blah, blah... disclaimer, disclaimer, disclaimer. :-P