An Opening Gambit

Once upon a time, I desired to play Dungeons and Dragons.
I had played once before, in a game my uncle ran for my brother and I. We spent one afternoon questing into the lair of a pirate gang.
That adventure, like so many others, ended before completion; that session was to be the only one of its kind.
But, my love of gaming was ignited.
I spent the next two years trying to play in RPGs. I pestered my brother for a session or two, to no avail (we took turns DMing a horrid combination of D&D 4e, 3.5 and 1e for each other until he got tired of it). I made characters for an Exalted game that my friend started, then ended, and a Battlestar Galactica RPG that the same friend started, then canceled, both before the second session.
Finally, I arrived in high school, with the fabulous, rumoured Dungeons and Dragons club.
This would be the promised land. This would be where I would find the campaign that would continue onward through the ages, would inspire many a story. This is where I would find the game that led me to my dream, that finally fulfilled my desire.

We all gathered there that one lunch period. The previous lunch we had sat in this same room, an empty computer lab with the curtains half drawn against the streaming sunlight, furiously scribbling letters and numbers on paper as we designed our characters from photocopies of a 3.5 Players Handbook. The poor illumination made my bespectacled eyes sting, but I cared not; this was the moment of glory. Our DM, an older student of nerdy aspect (let's call him Nick), sat across the table from us, his lunch left by the wayside. By the time we all were present and focused on this most momentous day, there were a mere twenty minutes left in the lunch.
"So. Let's start"
We all nod eagerly. We are a glorious bunch.
Myself, the full orc barbarian with a strength of 22 and an intelligence of 6. The sole female player, an elvish rogue who wished only to steal. The veteran player, a dwarvish binder who I pictured as the player, but with magic and a larger beard. The DM's friend, an odd kid who I remember nothing else about. The Role Player, a student one year older than me with a lightning wit, and a bard to match.
This would be our adventuring party. These would be our heroes.
"You're...umm...in a town. It's...somewhere?"
I exchange a look with the veteran player. Something is clearly wrong.
The DM scratches his head for a bit, then shrugs, and asks what we do.
As a group we deliberate, then head into the nearest bar. There, we hear a rumour about the fact that all the bread (or maybe it was beer) had been stolen from the town. We are asked to "please, go rescue our bread/beer". So began an exciting adventure of infiltration where the bard and rogue argued about who got to go first (and who got ogle whose butt), the binder and the best friend started down the tunnels, and I was declared too big to fit.
Then the bell rang, we cleaned up, and looked forward to the next session.

The next session began in that same room, with different people; the club had two DMs, and the other was sick. One of his members had joined us instead. The new player, a human swordsage, takes one look at the new dungeon we find ourselves in ("I got tired of the old one. It was lame" was the only explanation given for the sudden change), looked at the giant mecha who were walking around outside, and decides that the game isn't for him. Rather than say so, he has his character fall on his sword.
The DM shakes his head, and denies him the honour.
"You can't kill yourself! That's not how the game works. A magical shield comes out of nowhere, and protects from suicide forever"
The group was suitably unimpressed with that declaration, but, nonetheless, we continued on, and ten minutes later we have walked our way past the mecha to get to the town. The rogue (who by now we could tell the DM was crushing on), walks into the nearest weapons stores and asks for the biggest, bestest weapon. The owner announces it to be the Gatling Gun for a mere 5000 gold (far out of our resources).
But the player says "drat", and the DM, in his generosity, relents. 5000 gold rains from the sky, and she is able to purchase the gatling gun just in time to do battle with the dragon that has suddenly arrived. She is, in fact the only one equipped to deal with a monster.
The veteran player, new player, Role Player and I all look at each other. Even the DM's best friend seems put-off.
The bell again rings before we can finish anything.

The next session, I assume, began in that same room, with some of the same people. But, I was not one of them. I spoke to the other DM and the head of the club, and got transferred to a campaign that A) had some prep time put into it and B) made sense.

By January of that year the DM of the new group began missing session on a regular basis, being sick more often than not. Because our DM was such a good one when he was there, we still wanted to play, I took the helm with a dungeon I had sketched out in the back of my math notebook.
While this was my first time really DMing, I had already learned several important things from good old "Nick", from my uncle, and from the other DMs I had played (or not played) under.
I've listed these below as five lessons I learned from bad experiences, and five lessons I learned from good experiences. These lessons I still put into practice every single time I run a game.

Five lessons I learned from bad experiences.

1. Be consistent. If your players have come to expect something from you, don't pull the rug out from under them without good reason. "I got bored of it" is not a good enough reason.
2. Understand what your players want. If they want a serious game, give them one. If they want to go hunting the missing bread/beer, give them that. If you can't give them what they want, let them know, and either compromise, or suggest they find another DM.
3. Establish your campaign setting off the bat. In fact, preferably, do so before the bat. Let the players know exactly what they are getting in to.
4. Don't play favourites. That way leads anarchy, and dissatisfaction with the game (on your player's parts)
5. Let players walk away if they want to. Let player characters die if they must. It is the player's choice, and removing that freedom of choice is the same as removing one of the core reasons to play D&D.

Five lessons I learned from good experiences.

1. Don't be afraid to say no. At the same time, don't be afraid to say yes. The worst that will happen is you'll need to improvise. The best that will happen is that you'll all have a much better time.
2. An hour of prep time is worth far more than that at the table.
3. Being familiar with the rules is both important, and enjoyable. It allows you to make rules calls with confidence, shut down rules lawyers, and, occasionally, show off to the players. But don't do so too much, or else you won't be liked.
4. Half the fun of the game is the social face-to-face interactions. Don't try and stifle table talk all the time; sometimes it needs to happen.
5. A good story can come from anywhere. It can come from an offhand comment, a novel you once read, a nice piece of art in a rulebook...anything really. Don't be afraid to tell any story you want to (But see the second lesson above), no matter where it came from.