I give here a brief outline of the Word of Wisdom for those who would like to understand better this unique Mormon practice.
Where did it come from?
"When [the school of prophets] assembled together in this room after breakfast, the first they did was to light their pipes, and, while smoking, talk about the great things of the kingdom, and spit all over the room, and as soon as the pipe was out of their mouths a large chew of tobacco would then be taken. Often when the Prophet [Joseph Smith] entered the room to give the school instructions he would find himself in a cloud of tobacco smoke. This, and the complaints of his wife at having to clean so filthy a floor, made the Prophet think upon the matter, and he inquired of the Lord relating to the conduct of the Elders in using tobacco, and the revelation known as the Word of Wisdom was the result of his inquiry" (Brigham Young, Journal of Discourses, vol. 12, 158)
Like many revelations given in the Latter-days, this revelation came in response to a particular situation. Joseph Smith inquired of the Lord to know what to do and received what is recorded in D&C 89.
Why the Word of Wisdom?
Besides the obvious health concerns that it address, the Word of Wisdom was also given for another significant reason. "Behold, verily, thus saith the Lord unto you: In consequence of evils and designs which do and will exist in the hearts of conspiring men in the last days, I have warned you, and forewarn you, by giving unto you this word of wisdom by revelation—" (D&C 89:4). The Lord warns that there will be "evils and designs...in the hearts of conspiring men in the last days" and that is why he revealed this health code. It doesn't take much imagination to see such conspiring men in the tobacco and alcohol industries, does it? Here are groups of men getting rich of the lives, happiness, freedom, health, and families of those who consume and often are addicted to their products. The very definition, according to the Book of Mormon, of secret combinations are those who murder to get power and gain.
What about the health issues? It wasn't until over a century after the Word of Wisdom was revealed that science and society as a whole began to recognize the dangers of tobacco in particular. Medicine has also shown the benefits of avoiding alcohol, eating grains, and avoiding large quantities of meat. If this revelation were simply a product of Joseph Smith's time and environment, why has it been verified to a large extent by science only much later?
Early in the Church's history, Hyrum Smith announced that "hot drinks" meant "tea and coffee." Joseph Smith validated this interpretation. Some members of the church have interpreted that further to mean that caffeine should be avoided. However, the Church officially interprets it thus: "The only official interpretation of 'hot drinks' (D&C 89:9) in the Word of Wisdom is the statement made by early Church leaders that the term 'hot drinks' means tea and coffee" (LDS Church (2006). Church Handbook of Instructions, Book 1: Stake Presidencies and Bishoprics [Salt Lake City: LDS Church], 185). Tea has been defined as the drink that contains the actual tea plant, and not to include herbal teas.
Evolution of the Word of Wisdom
The Word of Wisdom was first understood as a guideline for members of the Church "not by commandment or constraint" (D&C 89:2) It wasn't until the early 1900's that the Word of Wisdom was reinterpreted or expanded by church leaders (particularly Pres. Heber J. Grant) to be no longer simply a guideline but a commandment from God. Since then, LDS members have been expected to live the Word of Wisdom in order to participate in temple ordinances and to advance in priesthood (for the males).
EDIT: Something else I forgot to mention. The revelation came, as I mentioned due to conspiring men that the Lord foresaw would exist in the last days. This leads me to believe that the principles of the Word of Wisdom are adapted for our time and place and we shouldn't expect to find them in the New Testament and perhaps some of the principles will change in a millennial day. The Word of Wisdom, in my mind, doesn't fall in the category of "never ending truth" but into the category of "this is best for you at this time."