There is some evidence that a vegetarian diet increases longevity, that a gluten free low-fat vegan diet can help with rheumatoid arthritis and vegetarians certainly fare well on coronary heart disease. (There's also evidence that the low-fat vegan diet helps with symptoms of type 2 diabetes, but I'm to lazy to go look up a good reference).
One interesting aside is this myth that early humans were predominately hunters this review article suggest that there is little evidence for the proportion of meat versus vegetables. (There is some evidence that modern hunter/gatherers who do incorporate a high percentage of meat do score well on diabetes risk indicators. However they were eating extremely lean meats.
Another article I stumbled across suggested the mediterranean diet is also shows effectiveness against certain diet related disease, but that diet includes <10% of calories from animals and >60% from plant sources. (And so isn't strictly vegetarian).
As far as I can tell the evidence is pretty overwhelming that "optimal" nutrition for humans seems to be based primarily on plant food sources. (Vegetables, Fruits, Nuts, Legumes, Whole Grains).
I've been going out to eat a lot more and eating cheese and more processed food, and wouldn't you know. I've been gaining all that weight I lost back. I really need to go back to eating "near-vegan". The problem is that's likely to make eating with my girlfriend much more challenging. (which is one of the reasons I've been getting lazy with my diet).