Italics on Wikipedia should be used sparingly, as described in Manual of Style sections "MOS:ITALIC" and "MOS:ITAL". Usage should follow "good English print style". What you'd expect in a book is what you should expect from Wikipedia.
Italics are generally used for emphasis and distinction. Examples include: when a word itself is being discussed, foreign words ("raison d'être"), titles of major works (including novels but excluding short stories), court cases ("Bush v. Gore"), scientific names ("E. coli"), and vessels like ships ("RMS Titanic") and trains ("City of New Orleans"). Of course, there are numerous specifics and exceptions.
Usually, quotations should not be italicized. There are also some cases where italics are used in a Wikipedia-specific way, such as disambiguation hatnotes on articles. Meanwhile, editorial usage of putting emphasis on words to indicate the potential for astonishment should be used very sparingly. After all, NPOV is always key.