Without getting dragged into a debate on the etymology of the word monk and stating up front that I've got no beef with the class, I'll try on the other shoe and say I can see where people who don't like the concept are coming from, and here are a few reasons.
Both conceptually and from a class-ability perspective, the class is obviously Asian in design. The word ki, a Japanese word meaning breath or spirit (or chi in Chinese). Many of the Asian martial arts incorporate the concept of of ki into their usage. Soft styles, like aikido and tai-chi go so far as to have the word in their name. Harder styles, like shotokan invoke concepts like the kiai shout (an active focus for your ki by shouting loudly at the moment a blow lands). Some of the abilities, abundant step being the most obvious, are references to myths about great Asian martial artists. Abundant step refers to limited "teleportation" abilities attributed to both Miyamato Musashi, Japan's reputed greatest swordsmen of all time, and Sensei Ueshiba, founder of aikido. However, I think it's the conceptual flavor-text that really throws people for a loop. The notion of seeking a true harmony of mind, body and spirit as a path to perfection is a very daoist concept. Daoism is, of course, an Eastern philosophy bordering on religion, from the Western point of view, originated by Lao-Tzu.
I think the issue is not so much that the monk class of D&D is Asian in feel inherently, but that the presented Asian concepts are so alien to many of the core concepts imbedded in D&D. Western philosophy tends to focus on achieving success/perfection via a single avenue. As someone earlier pointed out, western christian monks sought perfection of the soul through denial of the body. They fasted frequently, spent much time in prayer and meditation, engaged in self-flagellation to scourge the sin from themselves, etc. Alchemists and other scholars sought success/perfection via knowledge, and many westerners still prefer to seek such ends through the accumulation of material wealth or power over others. While these concepts are by no means foreign to the Asian region of the world, from a purely philisophical stand-point, Asian cultures tend to be amalgamationist in their views of human pursuits, espousing the notion that perfection is achieved through attention to all facets of life. This starkly counterpoints Western specializationism, where being the best at one thing is generally viewed as preferable to being passable at many pursuits. This is reflected in D&D's mechanics. Fighters focus primarily on strength and martial prowess. Wizards focus on knowledge as a road to power, while clerics find strength in devotion to a god and his/her causes. Rogues find success in clever application of skills and training. These are very Western concepts from which the monk class seems to stand apart.
My only personal gripe about the class is the conflict between their drive to find perfection within and 3.x's fixation on equipment. Are you really perfect in mind, body, and spirit if you require 200k+ gp worth of equipment to get there? It's actually the only aspect of AU's Oathsworn I really liked.
Finally, I think the monk fits much better in some campaign settings than others. You can obviously write up a homebrew where the class is either perfectly plausible or wildly out of place, but in terms of published settings, I'll echo a previous poster's sentiments in suggesting that Keith Baker found a very suitable, if narrow, niche for them in Eberron amongst the Kalashtar of Sarlona who live in mountainous monasteries (Tibet anyone) contemplating perfection and seeking to hold back the Inspired. However, they seem to stick out like a sore-thumb in FR, where the various contributors went so far as to include Middle Eastern regions, but nothing particularly Asian is present (except perhaps Rashemen).
Anyway, that's my two cents,
Z