I tend to agree with Zak's recent comment that "speed of response might just be the most important thing".
The DM sets the pace of the game. I learned this when I ran my first PBP game, but it's also true for in-person sessions. The game won't start until the DM is set-up and gives his introduction. Until then, people will just chat and catch-up almost indefinitely. And during the game, if the DM is constantly taking breaks to flip though a book, then you know he didn't prepare properly. As a teenager maybe you can get away with this, but when your game session starts at 9pm and you need to get up for work the next morning, this can really eat into adventuring time.
When preparing to run a session, one of your main goals should be, bringing down your response-time. Think about what monster stats, what charts, what non-combat mechanics you are most likely to need during the session, and bookmark them, or photocopy them or whatever. For mechanics you have never used before, now might be the time to run a little solo demo to make sure you understand the rules. At the very least, read through them before the session.
2. The Lull
This recent session I played in was a WFRP adventure which seemed, for the most part, to be scripted. The party starts out in town with a certain number of things they can do, and no jobs appear until they have wandered the town sufficiently(or something like that--I didn't actually read the adventure). Well, after about an hour of wandering, with all our attempts to look for adventure hooks answered in the negative, us players were starting to get pretty antsy. Finally the DM got the clue and jumped to the scripted adventure hook.
Now I'm not going to say this was a flaw with the module. The first part did build-up some background that came-out later in the session. But, on the other hand, this module was probably designed for a full-day session, not a 2 hour one like our group usually plays. Bottom line, we went an entire session without combat, and with only a bit of dungeon-delving towards the end.
Something similar happened in the recent sandbox session I ran. The players chose a more investigative route. I didn't see any problem with this, because they made the choice. But by the end of the session, they were also getting bored with no combats, while at the same time refusing to abandon their investigation.
So game sessions with no or sparse combat are boring, at least to my gaming group. What do I, as a DM, do about it?
One option is to prepare a fall-back encounter. Something where, if the game goes a half-hour without any combat, the DM pulls this one out of his pocket. This could be a pre-prepared combat. It could be a random encounter. Just something to liven things up when they get too slow.
In any case, as DM, keep your eye on the clock and your finger on the pulse of player interest. And be ready to abandon the script or abandon the sandbox tables and just throw something interesting when it's necessary.
3. Individual Player Interests
One more observation. Different players want different things out of the game.
For one player in my group, character advancement is very important. I have trouble relating to that. Personally, I'm more experiential--if we pulled-off a bunch of crazy stuff during the session, then who cares about XP. This player is the opposite. He's OK with grinding through completely dull encounters as long as he gets to mark advancement on his character sheet at the end of the session.
Now I'm not saying that the DM has to run-around trying to everyone individual want and needs. But it is a good idea to keep in mind what makes your various players happy, ESPECIALLY if their tastes differ widely from your own.