In my book, L is for one thing, and one thing only: LEVELS! But, how do levels play into a successful adventure. Well, when building an adventure, or a campaign, you have to always be aware of the level of your players and the levels of the enemies, at least for the purpose of balance, and making the encounters enjoyable and entertaining for your players (at least this is the accepted train of thought for 4th edition D&D). However, leveling up is an important thing to think about when you're plotting out a series of encounters. Knowing when your players are going to level up can make for a very rewarding tool; when you know that your characters are going to "Ding" you can scale up your encounters in advance accordingly. But, aside from just knowing when your players are going to level, you can be a bit more proactive with it. Let's face it, you're the DM, if you want your players to level up, then for god sakes level them up. I haven't had a lot of postive experience playing games at the lowest levels. That may just be my experience, but I prefer my characters to be pretty damn powerful. I think the same applies to most video games as well. In World of Warcraft you don't see players sitting around at the low levels on purpose very often. More often than not, players are going to power level as fast as they can in order to get to the top tier of the game where they can do some really, really epic stuff.
Power leveling is not really a component of most pen and paper RPG's, but there is of course a way to get around that. Power leveling is not in the hands of the players, it is is in the hands of the DM. Forcing a level jump, in which players skip several levels in order to get to a greater power point is a very easy way of mimicking the "power leveling" process. There are lots of ways to explain that story-wise; maybe the players take a break from adventuring to dedicate themselves to further training in order to hone their skills and develop new abilities. Maybe the players down something so incredibly awesome, or fulfill the end goal of a quest so epic, that they immediately go through a big jump in power. Go crazy. Just remember, if your players aren't interested in leveling up quickly, or would like to simply level their character though the natural organic process of killing monsters and looting their treasure, then let them. Don't get in the way of their fun. But, if they show interest in skipping the entire 1-20 and simply getting on with their really, really epic destiny than skip all the stuff in the middle and just go right there.