CNN.com has a nice article, including comments from Mike Nesmith, Peter Tork, and Ringo Starr.
For those who were wondering about his general health, the article also mentions that he was a vegetarian who ran miles on the beach every day. He was described as "not having an ounce of body fat on him." Apparently he was doing his best to take care of himself and the heart attack came as a shock to those who'd been dealing with him regularly. He was simply visiting his horses when he started complaining he was not feeling well and was short of breath. He went to his car to go home and was found there. EMTs got there quickly and he was taken to a hospital where he was pronounced dead.
Perhaps he simply had something wrong with him that was so rare that it went undiagnosed. That's what happened with figure skater Sergei Grinkhov, who suddenly collapsed at a practice for Stars on Ice. For that matter, I've known a couple of people who'd passed physicals, including stress tests, just fine and had a heart attack within weeks of the exam. One of them suffered a massive heart failure while at the Mayo Clinic while he was undergoing tests for something altogether unrelated and died instantly--and he was in his twenties at the time. Sometimes you just can't predict what's going to happen, no matter what you do.