Most Roguelike games take place in a fantasy setting, where you play a hero trying to save the world from Evil. To get to the Supreme Bad Guy, or retrieve the Ultimate Artifact, you have to wander through labyrinthine dungeons and towns inhabited by ferocious monsters, deadly traps, strange riddles, and magical items.
Sound familiar? Well, it is; Rogue was based on Dungeons and Dragons, and incorporates much of D&D's role playing flexibility as well as its character, monster, and item specification style.
Because dungeons tend to be mazes, it does not make sense to abstract or aggregate sections of a level into larger areas in which multiple maze sections (hallways and corners) or terrain features might be grouped together. Instead, each square (tile) in a Roguelike Game represents the smallest piece of viewable area. In a square can reside one player, one creature, one terrain feature. A corner in a hallway requires three tiles:
Since your character usually cannot see the entire dungeon level immediately but instead has to explore it using a light source (torch, lantern, infravision, etc.) with a set radius, you usually do not know which way a hallway will turn until you see that corner. Therefore, you could be walking east down a passageway (the '@' is you):
And as you take each step eastwards (each step corresponding to a keypress)...
You gradually uncover the layout of the maze...
# ####@# # #
Each step corresponds to a single action in the game world, a single keypress (unless you are using run mode).
# #####@# # #
This allows for exciting races between you and monsters...
D ######@ # ##
Especially when you are fleeing into unexplored territory.
# ######D # #@#
At at moment, you have no idea whether your next turn will uncover a dead end, or another monster lying in way. Each movement you make, each action you make, is critical.
# ####### # # #D@#
And you will know as soon as you make that action whether or not it was fatal...
# ####### @ # ##D#
...because each action corresponds to a turn (or more than one turn). You can never perform more that one action per turn. Likewise, you can never undo an action.
The dragon hits!
This immediate feedback is what makes Roguelike play dramatically different from that of other RPGs and especially from that of above-ground strategy games.
When the action takes place above ground, there tends to be lots of open space and no tight corners. Most if not all of the terrain on which a battle might be fought is visible, and there tend to be a myriad of movement paths from which to choose. Most of your time-critical action [e.g., fighting the enemy] takes place on familiar terrain, since engaging the enemy deep within unexplored territory is an invitation to a trap. The same high degree of visibility and multiple movement paths applies to the tactical [battle] screens on typical RPGs and some strategy games.
Therefore, in open-ground strategy games (and RPG tactical screens), movement tends to have a much smaller element of surprise attached to it. The scale of a game turn doesn't have to be very fine if enemy units are always visible before they get within one or two squares of you. Also, since in most turn based games the player is typically trying to coordinate the movements of multiple units rather than just one [which also allow a unit to be aware of enemy units far earlier than in a Roguelike game], orders are almost always issued in batches of actions rather than single actions. For example, each turn for a unit might consists of move multiple squares, engaging multiple enemies, moving and engaging, etc. Batching keeps the pace of a multi-unit game brisk.
The nature of dungeon exploration dictates many of the other characteristics of Roguelike games.
Roguelikes tend to involve dungeon exploration. Mazes have corners, which block line of sight. The reduced visibility allows the character to "practically bump into" enemy monsters before seeing them. Because the sighting radius is effectively one square, the distance a character (or monster) can travel in a turn is also one square. The pace of the game is therefore one action per turn.
You therefore usually only have one unit under your direct control. Otherwise, the frequency with which you would have to switch viewpoints would be absurd:
At the scale of one action per turn, having multiple units under [one or many players'] control would seriously bog down the pace of the game.
Also, since dungeons tend to be randomly generated mazes instead of fixed locales, a visual (ASCII graphics) interface is more appropriate than a purely prose-based (interactive fiction) interface. This random element additionally greatly boosts replayability; when's the last time you played an static-world adventure game ten, twenty, a hundred times?
Also, as I mentioned earlier, dungeon crawling [killing monsters in mazes] comes mostly from Dungeons and Dragons. D&D players tend to be highly creative as a hobby; D&D players and especially dungeon masters (referees) often spend hours of their free time creating characters, dungeons, rules, etc. To them, the labor of creation is fun in itself. When the DIY D&D types get hold of computers, their natural instinct is to learn how to create characters, dungeons, and rules on the computer. They slave long hours to create games, and do it for fun instead of money. This probably contributes to most Roguelikes being freeware.
D&D players are also accustomed to dealing with abstraction, relying on their imagination to paint the details on the monsters and terrain. This is probably one of the main reasons why D&D players who get into creating Roguelikes don't mind (and often prefer) sticking with ASCII graphics. They'd rather tell the player, "you see a dragon fifty feet to the north" instead of "you see my painting of a dragon standing in my painting of a room fifty feet to the north."
So, to sum it all up, the dungeon crawl nature of Roguelike Games has contributed to the following characteristics:
There are, of course, exceptions to the dungeon crawling characteristic. Ragnarok takes place mostly above ground in wide spaces (at least, I never got far enough into the game to see more than a couple hallways). I've also seen a lot more wilderness travel than dungeon exploration in Unreal World and AlphaMan.
Even Roguelikes in which case most of the time is spent in the dungeon also have some sort of wilderness or non-maze location. Moria and Larn and their descendants have always had a town level, and many Angband variants now have a large, semi-random wilderness area. NetHack has open-spaced quest levels and semi-random town levels, though during the course of an average [short-lived] game you might not see either of these. ADOM and Omega each have static towns and wildernesses, and ADOM (and possibly Omega) has wilderness encounters, in which a random tactical map is popped up. Usually, however, most of the combat in Roguelikes takes place down in the dungeons.