LSD is illegal without medical supervision in most or all Countries. With "responsible" use it has no addictive properties but LSD use can lead to use of other more addictive drugs. Amy Winehouse blamed her addiction to Crack cocaine onto a Hallucination of the Devil which she suffered under LSD.
Bad trips can cause great harm, Small numbers of people have done silly things like jumping out of high windows under the influence of LSD. It is worth noting, however, that a far larger percentage of Alcohol users put themselves at risk while through unsafe activities). To prevent the remote possibility of any potential injuries or logistical complications while under the influence of LSD, it is suggested to have a sober "trip sitter" present - preferable one who is experienced in dealing with "tripping" people. According to The Lancet LSD is less harmful than Tobacco or Alcohol. 
LSD works primarily by binding to the 5-HT2-A receptor. Subsequent changes cause communication between areas of the Brain that would not otherwise occur and also reduces dialogue between other areas. The subsequent effects of these changes may include feelings (or illusions, remember the brain isn't working normally) of insight into the workings of the self, society, nature and the Universe, profoundly spiritual thoughts and feelings (mystical experience), blending of the senses (synesthesia), euphoria/dysphoria, paranoia, fear, panic, open and closed-eye visual imagery, recovery of old memories, and other stuff. Flashbacks occurring due to a previous bad trip are a rare but serious side effect.
LSD has taken a negative hit from Christian zealots because it has been shown to manifest what enthusiasts call, "authentic religious experiences", therefore posing a threat to the world's religious infrastructure.