One thing I've thought about in preparation for the Word of Wisdom I taught today is the concept of sinning and ignorance. Before the bretheren received the Word of Wisdom, many of them used tobacco, and possibly even drank alcohol occasionally, and this both was and was not okay.
Sin has at least two kinds of consequences: eternal consequences and temporal consequences. If one is sinning in ignorance, the eternal consequences will largely be mitigated. God doesn't punish people for breaking commandments they don't know about. If a person doesn't know that it's a sin to play sports on Sunday, God's probably not going to be too hard on them for doing so. So, unless I'm wrong, sinning in ignorance doesn't carry very many or very strong eternal consequences.
The temporal consequences, however, remain fully in force. The early saints, or anyone else who used tobacco before the Word of Wisdom was revealed, may not have been punished for it, but the tobacco would still have had negative effects on them. It still would have been harmful and potentially habit-forming. Thus, even though their eternal welfare wasn't at stake, it was still good for the early saints to learn about the dangers of tobacco.
God's commandments aren't just rules that God punishes us for breaking. They're warnings about things that are inherently harmful to us. Not knowing about those commandments might spare us from the eternal consequences of ignorantly breaking them, but it won't spare us from the temporal consequences of those actions. Sinning in ignorance isn't as bad as wilfully sinning, but that doesn't make it harmless. There are consequences for sinning, even if a person doesn't know they're doing it.