The issue of drugs and alcohol is far more complicated than some of the simplistic views mentioned here.
In many ways, the fact that alcohol is legal while marijuana remains outlawed is ridiculous. Both can have severe long term effects, but in the short run, alcohol can do much more damage emotionally and even physically. In moderation, marijuana isn't always all that dangerous. One misconception, however, is in the fact that today's weed is often compared to the weed of the 60s and 70s that our parents smoked. Today's marijuana is FAR more potent than what mom and dad had, and weed can often be laced with chemicals and more dangerous drugs. Legalizing marijuana would most likely eliminate this threat, however, as consumers would know what they're getting if purchased from an 'authorized' dealer. The fact that marijuana is illegal for the vast majority of us hasn't stopped those who want to use it do so.
As far as pot being a "gateway" drug, I have to agree. I've had friends move on to other drugs after simply becoming bored with marijuana. After a while, it takes more to get the 'same' high, and this is when things become dangerous. In the end, much of it is about moderation. Abstinence is obviously the best way to remain safe, but those who choose otherwise are making a decision that shouldn't necessarily be looked down upon.