Cliffnotes on vegetarianism: Yes, you should eat less meat and more veggies.

Ever since I read the 1999 meta-analysis of vegetarian diets and mortality (2) I wanted to see a follow-up. Finally, some epidemiologists gave it another try.

Enter Huang et al. 2012 (1):
Seven studies with a total of 124,706 participants were included in this analysis. All-cause mortality in vegetarians was 9% lower than in nonvegetarians (RR = 0.91; 95% CI, 0.66-1.16). The mortality from ischemic heart disease was significantly lower in vegetarians than in nonvegetarians (RR = 0.71; 95% CI, 0.56-0.87). We observed a 16% lower mortality from circulatory diseases (RR = 0.84; 95% CI, 0.54-1.14) and a 12% lower mortality from cerebrovascular disease (RR = 0.88; 95% CI, 0.70-1.06) in vegetarians compared with nonvegetarians. Vegetarians had a significantly lower cancer incidence than nonvegetarians (RR = 0.82; 95% CI, 0.67-0.97).”
"Begg’s funnel plot and Begg’s test showed a slight significant publication bias in all-cause mortality, cancer incidence, ischemic heart disease and circulatory disease. No publication bias was observed in cerebrovascular disease."
Nonetheless the risk ratio for cerebrovascular disease is not materially different from the other risk estimates, arguing against the null hypothesis. What this may suggest is inadequate statistical power.
And there was some heterogeneity between studies
and residual confounding or bias always remain a big problem in observational studies of this sort. However, sensitivity analysis showed that heterogeneity did not unduly influence the data. 

If it were not for supportive evidence from other study designs, including well designed controlled trials, I wouldn't consider this study to be all that solid evidence. But we do have evidence that:
  • dietary patterns like "prudent", Mediterranean, vegetarian + (marine) n-3
  • food groups like legumes, vegetables, fruit, nuts, olive and n-3 rich plant oils, whole-grains
  • dietary substances abundant in plant foods like fibre, potassium, magnesium, vitamin K, antioxidants, plant protein, secondary plant metabolites, etc. are healthy
and that plants lack many detrimental substances found in foods of animal origin like heme iron, saturated fat, cholesterol, toxic compounds formed due to cooking and heat, BCAA & methionine rich protein.

NB: the above is a simplification and much more could be said on the topic. There is evidence that a vegetarian diet can be further optimized by including low fat dairy and either a source of marine n3 fatty acids or regular intakes of ALA (=n3 of plant origin).. that lowish intakes of white meat can be acceptable... that vegetarian diets can be improved by supplementation and much more.

(1) Ann Nutr Metab. 2012;60(4):233-40. doi: 10.1159/000337301. Epub 2012 Jun 1.
Cardiovascular disease mortality and cancer incidence in vegetarians: a meta-analysis and systematic review.
Huang T, Yang B, Zheng J, Li G, Wahlqvist ML, Li D.

(2) Am J Clin Nutr. 1999 Sep;70(3 Suppl):516S-524S.
Mortality in vegetarians and nonvegetarians: detailed findings from a collaborative analysis of 5 prospective studies. Key et al.