There has never been a reported death attributed directly to marijuana intoxication. The key word there is obviously directly. It can indirectly cause accidents and other events that could result in death due to judgment impairments caused by intoxication. To my knowledge (and I would have to do a current literature search to verify) that there has not been a direct causal link between marijuana and lung cancer. This could be due to the fact that many people who smoke marijuana may also smoke cigarettes or do other drugs that may cloud the picture greatly. Also, in the vast majority of cases marijuana users only use the drug for a short time often during their youth whereas cigarette smokers commonly smoke until they die or are truly unable to smoke due to incapacitation secondary to disease. This time is often 50+ years. That being said I do believe marijuana has a detrimental effect on a patient's respiratory health even if it is negligible compared to cigarette smoking.
One link that has been researched is that the incidence of schizophrenia is higher in males with a genetic predisposition when the drug is smoked in adolescents. It also can cause depression and what is known as demotivational syndrome.