It's one of those cookie-cutter ballparks from the 1950s and 1960s. Baseball was expanding at a fairly rapid rate, and no one felt like designing a ballpark that was new and different. Fenway Park and Yankee Stadium have unusual dimensions, but Shea and others built in that era are all around 325 down the lines and 400 to center, and perfectly symmetrical. Blah.
Having said that, there are some nice things about Shea Stadium. Their scoreboard is first-rate. It has the usual stuff about who's batting and who's pitching, but it also lists both team's lineups all the time. A nice touch. There is also an upside-down top hat over the center field wall, that has the Mets' logo on it. Whenever a Met hits a home run, a big apple saying "New York" pops out of the hat. It's fun.
Actually, the Giants don't use Shea Stadium. They use the Meadowlands, along with the Jets. It's where Jimmy Hoffa is said to be buried (although he probably isn't).