One of my favorite encounters is the Kuo-Toa ferryman offering passage across the underground river in module D2: Shrine of the Kuo-Toa. The encounter is a gem-like example of the admirable qualities of early D&D. (Please note spoilers follow, if you wish to play D2 in the future.)
Evocative: Sets a feel of gonzo, surreal weirdness. As written in 1978 this creature was the first Kuo-Toa encountered, so is strange, exotic, and horrific. Yet when he approaches, it is for mundane purpose: to croak out in the eerie underground language the price for passage across the river. Imagine encountering a lovecraftian fish-beast lurking in the dark near an underground river who opens his mouth and says… “Do you want the blue plate special?” The overall effect communicates loads of atmosphere.
Non-prescriptive: The players get what they bring to this encounter. As Gygax says in the introduction to the module, “the river crossing, can be very easy to accomplish, or the rash party can turn it into a deadly nightmare.” The result of meeting Thoopshib could be anything from peaceful transaction to a nasty combat and/or an accidental raft trip all the way to the Sunless Sea – largely driven by how the characters respond to the situation, not a pre-ordained script. Yet the likely outcomes are reasonable based on party actions.
Random element: Thoopshib is unbalanced, and if he is not understood he has a chance of going berserk. The chance of him flipping out provides both an element of surprise for the DM, and a layer of challenge for the players – have they realized they need to be able to communicate with the denizens of the underworld? Have they secured a translator during their journey so far? Even so, the situation could still turn bad – welcome to the underworld! (Note this represents an elegant solution in this encounter to analysis/paralysis – the longer the players dither over how to deal with Thoopshib, the more likely he is to go off.)
For those who like literary and story-telling elements in their adventuring, the encounter is foreshadowing (and metonymy, for you english majors). This simple encounter is at heart the whole module writ small: an encounter a savvy party can simply walk through, but a combative or greedy party can founder on. Thoopshib offers a very topical lesson to the “rash party” capable of learning from experience, right before they walk into the Shrine. In writing this is known as “show, don’t tell.”
Concise: In less than half a page, 600 words, EGG outlines a robust encounter, limning the situation such that a DM can fill in details, adapt the situation to a particular campaign, and respond to a wide variety of player actions easily, all without losing the general outline or purpose of the encounter. Like with many (but not all!) of the D&D ur-texts, there is a lot of content and little wasted space. (See “Evocative” above.)
1) [SPOILER] For those not familiar with the encounter, it occurs at a river crossing along the shore of a vast underground river. Thoobshib is an “unbalanced” Kuo-Toa who charges a fee to pole passengers across the river on his barge. He offers to ferry the characters across, speaking in the common tongue of the underworld. Each time he has to repeat his offer he has an increasing chance of going berserk and attacking. He is a formidable creature and even a large and powerful party appropriate to the module (at least six players, average ninth level) will have trouble if they do not handle him well.
 In sharp contrast to later D&D products (I am looking at you, Dragonlance) the characters could actually be completely sidetracked out of the rest of modules D2 and D3 if they are not careful, in a sort of anti-railroad.
 I wish I could write like that. Instead, my little review here has used 663 words and three footnotes! to describe 605 words of encounter. For shame! ; )