JOEL - The Word of Wisdom has been interpreted to include hot coffee or black tea. Joel H. Johnson, with whose family the Prophet Joseph Smith was intimate, relates that on a Sabbath day in July (1833) following the giving of the "Word of Wisdom," when both Joseph and Hyrum Smith were in the stand, the Prophet said to the Saints: "I understand that some of the people are excusing themselves in using tea and coffee, because the Lord only said 'hot drinks' in the revelation of the Word of Wisdom. Tea and coffee are what the Lord meant when he said 'hot drinks.'(Johnson, J. H., A Voice from the Mountains, p. 12)
Anything besides coffee and black tea is left up to the judgement of the individual. However, we have been advised by our modern-day leaders to not drink anything containing harmful drugs(including caffeine), in a way that would result in a habitual use of the drug (1972 letter from First Presidency). Decaffienated coffee still contains small amounts of caffeine, so this is why many will say that it should be avoided as well. In fact some medical investigations over the last decade have shown that between 40 and 50 percent of decaffeinated coffee drinkers have gastrointestinal difficulties, such as ulcers, colitis, or diarrhea. Chocolate also contains a form of caffeiene, but I can promise you I have no plans to give that up. In fact Elder Mark E. Peterson once said:
"At no time has cocoa or chocolate been included in the prohibitions of the Word of Wisdom, and at no time has the Church said that cocoa is as harmful as coffee. Those who make these claims do so on their own responsibility, and obviously without knowing the facts of the matter" (Elder Mark E. Peterson, Patterns for Living [Bookcraft, 1962], pp. 235-37).
Just to make life easier on myself, I avoid all forms of hot or cold, regular or decaffeinated, coffee and black tea, and caffeinated soft drinks. But this my own opinion about how I have decided to live the principle.
Perhaps you can make the judgement yourself. Is your decaffeinated coffee and tea something you have to drink to get through the day, or could you easily drink something else instead? Are you receiving the blessings of health promised in the scriptures(D&C 89); about being able to run and not be weary, and walk and not faint? If you are not, you may need to reconsider what you take into your body.
We have also been told that we should "abstain from all appearance of evil" (1 Thes. 5: 22). A non LDS person might think of you as a hypocrite if they see you drinking what looks like real coffee.
Once again, and of course this is my opinion, the judgment of what you should or should not drink is up to you, about anything other than regular hot coffee and black tea. If you feel that you are living the Word of Wisdom in such a way that maintains your body as an acceptable temple for the spirit of the Lord to dwell in, then you should be able to respond affirmativley to the temple reccommend question about obeying the Word of Wisdom.