Marijuana has less of a potential for addiction than alcohol or tobacco. This is not to say that you cannot become an addict (I've met a few) but the habit is more difficult to acquire and easier to kick than the other two. Alcohol also stimulates aggressive tendencies more than marijuana or just about any other depressant (which is a kind of a paradox, once you put some thought into it).
Exctacy is extremely dangerous for those who take it. I have seen a guy who popped a knee and kept on dancing and jumping around. They had to get the ambulance for him the next day. However, it's not widely available, so it has less of an impact on the society in general.