What would be the major changes to our bodies if we were herbivores? Would our bodies be weaker because they were not getting any protein from meat? Would we be completely different? (Probably, I suspect).
As far as I can tell, the major changes would be to the human digestive system, as well as to all body parts involved in processing food.
The teeth of carnivores and herbivores differ because they eat different things, which require different techniques to chew. Plant matter is a lot different in consistency when compared to a slab of meat. Wikipedia has a nice note on the subject:
The shape of the animal's teeth are related to its diet. For example, plant matter is hard to digest, so herbivores have many molars for chewing and grinding. Carnivores, on the other hand, need canines to kill prey and to tear meat.
Our teeth are best suited to eat both meat and plants; if we became herbivores, I would expect that our molars would develop even more, while our front teeth would become less developed.
As you mentioned there are conspicuously low levels of protein in plants. Herbivores, as a whole, will most likely receive a different set of nutrients than carnivores. Stomach enzymes and fluids work to break down different substances; we would see a change in these enzymes and acids as more plant matter was introduced into our diet.
If Wikipedia is correct, the major protein-processing enzymes include proteases and peptidases. We might see reduced numbers of these in a group of herbivorous humans. These acids are active all over the digestive system, so other organs would be affected
Would our bodys be weaker because its not getting any protein from meat?
Have you ever fought a 400 lb gorilla? Exactly.
Humans are omnivores, this has resulted in our digestive system evolving as it has. However, if humans had evolved as herbivores, then the following changes would be necessary:
Human teeth are actually fairly well adapted to a herbivorous lifestyle already, albeit herbivory that includes mostly fruit and nuts and soft leaves. Were humans to evolve as leaf/grass eaters (lower quality food), teeth are worn down rapidly by such a diet, and the molars would have to be larger and would either have to be replaced frequently (as in elephants) or be open-rooted so as to grow continuously (as in horses). The incisors may also have to be open-rooted, depending on if they are used to strip leaves off branches, or if they are used to crop grass. The numbers of teeth - and the size of the mouth - depends on the method by which the diet is digested.
A herbivorous diet with lower-quality food requires a considerably higher amount of treatment in the gut than human's current diet. There are two ways this can occur:
Foregut fermenters (such as cows) have large stomachs in which their food is stored in a bacterial soup which aids its digestion. They are able to regurgitate their food in order to chew it later. They have long intestines to maximise the digestion of the pre-treated fodder. However, if they eat food that is too rich, they tend to suffer from bloat as the foregut bacteria metabolise and reproduce out of control.
Hindgut fermenters (such as horses) have normal sized stomachs, long small intestines, and an enlarged caecum (that in humans is the appendix) before the large intestine. Hindgut fermenters must chew their food more thoroughly (and must therefore have more grinding teeth, hence the answer to the question about horses, "Why the long face?") than foregut fermenters. Partially digested food accumulates in the caecum, where it undergoes bacterial fermentation, however, high quality food is not an issue, since the pre-treatment in the foregut would have removed the high-quality components, leaving only the difficult-to-digest stuff. However, hindgut fermenters are not quite as efficient as foregut fermenters. (As a diversion, this is why horses can be fed apples, but cows cannot.)
A human-sized herbivore would likely gain more advantage from a foregut-fermentation digestive system, as it would not require as great a change in dentition, however a hindgut fermentation system would allow a greater variety of food. Gorillas, who are primarily herbivorous, are hindgut fermenters.
OK, it looks like nobody mentioned one major difference.
Research indicates that human brain sized increased very rapidly as our ancestors switched from a primarily herbivore diet to including meat in their food.
Meat eating means hunting. Hunting means group activity. Group activity means communication. Communication means sending and receiving complex detailed informational messages. Detailed messages in short time mean the need of producing many types of sounds. Many types of sound production means you need a larynx not deep down your throat like the gorillas have, but right up up up at the top of the throat the way we have now.
There are several other important changes related to meat diet, but most of them are about the psychology and social structures of early peoples, so I will not mention them here.