Common St. John’s wort is easily recognized, with the plant’s distinguishing characteristics including five yellow petals (often ringed with black dots), opposite leaf arrangement and — the reason for the epithet perforatum — leaves that are pellucid. Held up to the light, the leaves appear perforated, though the “holes” are actually transluscent oil glands.
Some of what was written yesterday about foxglove could be applied to common St. John’s wort: a species native to Eurasia that has been widely introduced elsewhere, including North America, South America, Australia and South Africa; a species fatal to livestock; and a medicinal use for humans.
Hypericum perforatum has been so successful upon introduction into new areas that it is considered a high-risk invasive plant in some jurisdictions. In typical invasive fashion, it can form dense stands (scroll down to see related photos) and crowd out native plants. This quality combined with the fact that it is poisonous to livestock (it can induce mania and depression, act as an abortifacient, cause dermatitis and lead to death) helped it to develop a reputation as a hated weed.
That reputation has been somewhat softened in recent years with the confirmation of its efficacy as an antidepressant in humans. Wikipedia and the Plants for a Future database provide good summaries of current and historical medicinal use in humans.