One of my favoritest mechanics in D&D is the wandering monster. For starters, it combines two great D&D elements: the stupid random die chart and monsters. (I know not everyone digs random charts, but who doesn't like monsters?) The random factor forces the DM to think on his feet. What the hell is that troll doing on level three, if the troll lair is on level five? Why is there a flumph in the dungeon at all? You don't always have to have an answer here, but for me half the fun of DMing is riffing off of random crap like that.
Moldvay Basic D&D does a great job putting nifty new game elements on the table because of all the human-type people that appear on the dungeon charts. Why are there d8 Traders wandering level one? Are those the jerks who keep selling oil and poison to Tucker's kobolds? That Noble and his retinue you bumped into on level two, what the heck is his deal? He can't be up to any good.
And once you get talking about people, that leads directly into one of the greatest banes of old school dungeoneers: the NPC party. In my experience few things rile a group of players more than the idea that some DM-run goons might get to all the treasure first. As an aside, on more than one occasion I've rolled up an NPC party and solo-ran them through a dungeon between sessions. "Here's another room with nothing but hacked-up monsters and empty chests. What the hell is going on here? Who is stealing all our xp and loot?"
Wandering monsters help keep the players on their toes and the game moving at a brisk pace. Competent players will quickly realize that if the DM is rolling wandering monsters every turn or two then they need to use their time wisely. Sure, you can search every stinkin' inch of the dungeon for secret doors, but how many wandering monster checks will the DM make while you do it?
Tomorrow I'll talk a little bit about the tricks I use when I gin up a new custom wandering monster chart.