Despite what the above meme says, there are actually a variety of reasons I'm not a Mormon. Mormonism has many fatal errors, most of which are far greater than its opposition to coffee. However, because God is perfect, even a small error is enough to prove that an entire religion is not from God. For this reason, even if Doctrines and Covenants 89:9 was the only error in Mormonism, it would still be a good enough reason to reject it.
Christian liberty is a somewhat unique concept among religions. It is a very free faith, with the general concept being that if something is not explicitly forbidden, it is implicitly allowed. There are exceptions, such as cannibalism, which is not explicitly forbidden, but is forbidden implicitly by the fact that it is only ever mentioned as a terrible terrible curse, and we are explicitly told that man is made in the image of God. However, generally, an explicit command is required to call something sinful.
According to basically everything in the New Testament, there are no rules for food and drink. Romans 14 calls arguments over food and drink "doubtful disputations." Colossians 2:16 tells us to let no one judge us in food or drink, and 1 Timothy 4:1-3 tells us that forbidding certain "meats" (which, as shown by Genesis 1:29, is used by the KJV to refer to that which is consumed, and does apply to plants, such as coffee) is a "doctrine of devils".
Doctrines and Covenants 89:9 tells us that hot drinks are not for the body. Now, personally, I like my coffee so strong, it comes with a warning label. Clearly, hot drinks are often used for my body, so I feel rather judged by that. Immediately, therefore, Joseph Smith has violated Colossians 2:16 by judging people, like me, in our drinking habits. Strike one. This misapplied judgement is a part of the Mormon religion, which of course causes disputes. I remember specifically asking a Mormon to tell me a random command so I could make a point, and he immediately chose the prohibition on coffee. Therefore, Joseph Smith has caused a "doubtful disputation" in violation of Romans 14:1. Strike two. The very name "Doctrines and Covenants" tells us that this Mormon scripture is intended to be rather authoritative. Specifically, the prohibition on tea, coffee, hot chocolate and any other hot drinks is a doctrine that forbids certain "meats", which 1 Timothy 4:1-3 tells us is a doctrine, not of God, but of devils. Strike three and he's out.
During their lives, prophets made a wide variety of mistakes. Of course they did, they're human. However, none of these mistakes became a part of the Christian religion. Quite the contrary, these mistakes were often used to demonstrate the solution to them. Moses, for example, chose to strike the rock, giving the illusion that he was the one who brought the water to the Israelites. This was a major mistake on Moses' part, but he was punished for it. But Joseph Smith's mistake regarding hot drinks became a part of Mormon doctrine. To this day, the Mormon religion forbids its followers to consume hot coffee.
Joseph Smith violated the Scriptures in a number of ways. This one little verse in Doctrines and Covenants is an affront to the liberty Christians enjoy by the grace of God, and through it, Joseph Smith revealed himself as a false prophet.