Tomb of Horrors is one of those modules that is spectacularly misunderstood by those who never played or ran it, and those who began playing Dungeons and Dragons after the advent of 2nd Edition AD&D.
That the Tomb of Horrors is misunderstood will come as little surprise. The infamy of Tomb of Horrors, along with that module's relative scarcity to the uninitiated (at least prior to the advent of usenets, eBay and other resale markets), means that this module has become surrounded with a mythology not supported by the facts.
Both oblivious and informed role-players regard Tomb of Horrors as a killer dungeon. Absolutely true. But it is a killer dungeon almost entirely bereft of monsters. Tomb of Horrors is a killer dungeon not because of fell beasts, but because players fail to attend to the clues provided via the boxed text and module illustrations.
In the module's introductory paragraphs, Gary Gygax has this to say about the Tomb of Horrors:
"This is a thinking person's module. If your group is a hack and slay gathering, they will be unhappy! It is this writer's belief that brainwork is good for all players, and they will certainly benefit from playing this module, for individual levels of skill will be improved by reasoning and experience. If you regularly pose problems to be solved by brains and not brawn, your players will find this module immediately to their liking."
The uninitiated often assume that only uber-powerful characters, loaded to the rafters with magic weapons, items, spells, and skills, can last for more than a few seconds in, and successfully navigate through, the Tomb of Horrors. The truth is that it is not magic and CHARACTER skill that is needed to complete this module: rather, it is PLAYER skill. You get no perception checks. No daily powers. No pushes, pulls, marks and combat synergies. Just good old-fashioned player cleverness and problem-solving.
You can potentially complete the Tomb of Horrors without once engaging in combat, if you are careful and observant. Even the ultimate encounter with the Demi-Lich, Acererak, can be completed without a single spell thrown or weapon unslung:
"All that remains of Acererak are the dust of his bones and his skull resting in the far recesses of the [treasure] vault. If the treasure of the crypt is touched, the dust swirls into the air and forms a man-like shape. If the shape is ignored, it will dissipate in three rounds, for it can only advance and threaten, not harm....[however] if any character is so foolish as to touch the skull of the demi-lich, a terrible thing occurs..."
There is no need to kill the end boss guy in the Tomb of Horrors. In fact, if you choose to engage the skull of Acererak, you are more likely that not to be on the grinning-skull end of a Total Party Kill.
To defend the artificial nature of modern D&D mechanics and adventures, some have argued that the Tomb of Horrors is equally artificial, as the Tomb exists merely to be looted, just as modern adventures exist merely to award accolades and experience for monsters killed, quests completed, and synergies optimized. To argue thusly is to completely misunderstand what original and early versions of Dungeons and Dragons is about. Original Dungeons and Dragons was designed to allow players to emulate the sword & sorcery fiction genre, without a script. There is no predetermined plot in the Tomb of Horrors; no story; no lofty goals. Only those created at the game table by free agents (players and DM alike), rather than ones foisted upon the participants by the adventure's author.
As for the metagame argument that Tomb of Horrors exists for the sole purpose of being looted, that is also true. In early versions of Dungeons and Dragons, experience is earned primarily through the recovery of treasure. Just like the characters in sword & sorcery literature, those characters are no heroes: they are mercenaries and grave-robbers. Their motivation for entering the Tomb of Horrors is wholly transparent. They are looking for loot, and have heard that it can be found in Acererak's Tomb.