MSG is a strange product as it doesn't taste Sweet, Sour, Bitter or Sweet which are the 4 main areas that our tongue breaks down tastes into. Instead it is the essence of the 5th (relatively unknown) taste Umami.
Courtesy of Wikipedia:
Umami is one of the five generally recognised basic tastes sensed by specialized receptor cells present on the human tongue. Umami is a loanword from Japanese meaning roughly "tasty", although "brothy", "meaty", or "savory" have been proposed as alternative translations.[1][2] The same taste is also known as xiānw?i (traditional Chinese: 鮮味; simplified Chinese: 鲜味 literally "Fresh Flavor") in Chinese cooking. In as much as it describes the flavor common to savory products such as meat, cheese, and mushrooms, umami is similar to Brillat-Savarin's concept of osmazome, an early attempt to describe the main flavoring component of meat as extracted in the process of making stock.