What's the major harm in smoking weed?

None, if . . . . No harm at all, if you don't mind distorted perceptions, impaired coordination, plus difficulty with thinking, problem solving, learning, and memory. This adverse effect on intellect can last for days or weeks after the drug wears off. If you also don't mind potential paranoia & psychosis; lung irritation; exposure to carcinogens; heart palpitations; and oligospermia, it's your choice also.
Psychological SE's. Possible effects of marijuana: anxiety, depression, fear, panic ,delusions, paranoia, hallucinations, losing touch with reality, inattentiveness, short-term memory problems, disturbances of thought processes. Fatigue, poor concentration, slowed reaction times & slowed problem solving.