More than five years after Palmer residents rejected a casino proposal for a huge tract of land just off Turnpike exit 8, the property is back in the news, this time as the planned site of a $650 million water park, resort spa, and sports complex. It’s the most basic tenet in commercial real estate. Location, location, location. Since the Massachusetts Turnpike opened in 1957, the large tract of land sitting atop the hill overlooking the exit 8 ramp in Palmer has always possessed that coveted quality. But over the ensuing 60-plus years, little has been done to capitalize on it.