People like the Japanese that have diets very high in vegetables and fish tend to live longer and have less chronic illnesses. Meat is not bad for you, but the quality of the meat (grass fed organic is best) and the overall ratio of fat/protein/carbs is important and of course, your activity level is another factor.
Being a vegetarian or a vegan doesn't guarantee you are or will be healthy; it's a combination of wholesome, minimally processed foods 80-90% of the time and exercise that lay the foundation for good health.
I find that the healthiest diet is one of moderation. If you eat real food most of the time, there are many combinations that will yield the same results whether it be plant heavy or meat heavy.
Here are some good reads:
http://www.sciam.com/article.cfm?id=...t-of-longevity
http://www.livestrong.com/health-art...-071cd22b3d86/