Despite the way a lot of people view Frank Mentzer's Basic Set for its hand-holding tone directed at 10 year olds, I've said it many times that as an 11 year old with little experience in fantasy gaming other than Choose-Your-Own-Adventure and Endless Quest books, that tone was just what I needed to get me into the game for life.
There's also something to be said about how the box sets of BECMI break down the tiers of play.
Start out with dungeons. Clear premise (monsters and treasure in dangerous ruins). Focused game play.
Expand to wilderness and town adventures after you've gotten used to dungeoneering. Explore the world, go on quests, become more powerful.
Once you've gotten enough dungeon/wilderness exploration done, you build a stronghold and become a ruler. Deal with political factions and grow your power even more.
There are also epic quests to go on, including to the other planes of existence or searching for extremely powerful artifacts.
Get powerful enough, you might challenge yourself to become one of the deities.
Become a deity, do god stuff. Maybe decide to become a mortal and do it all again?
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But despite this obvious progression, there really are some holes in published modules, aren't there? Plenty of dungeons at all levels. Lots of wilderness adventures for low to mid level. A good number of epic quests for artifacts for high levels.
But not so many town/city adventures. Very few domain game modules (CM1 Test of the Warlords is the only one that springs to mind, outside of the Birthright AD&D stuff). And not a lot of planar adventures. And a lot of the planar adventures are either a brief jaunt into another plane, or a trip to an alternate Prime Plane. Not a lot on the elemental planes, astral plane, or most outer planes. Just lots and lots of dungeons.
It's no wonder the people at WotC decided to focus all their efforts on games of never-ending but progressively harder (but not really because you level up with them) dungeon adventures.
