From my own vegetarianism, and temporary veganism; the problem with it is the high consumption of carbohydrates. Plant based proteins, don't contain the same amount as their meat counterparts.
A huge reason why I stopped being a vegetarian is due to doing some reading on the defense of plants against those that ate them --- in cafe man/scavenger times it was simply safer to eat meat (it being properly cooked, of course) due to plants either being poisonous or containing a harmful ingredient that will make you feel like shit.
What I am trying to say is what heavily turned me was doing some reading on the "poisonous" ingredients in fruit seeds, which I originally thought was lecithin --- it's not. Here's a link though
http://www.livestrong.com/article/188977-apple-seed-toxicity/
If you can eat dairy, it's very possible to eat a high protein diet. I have a friend who sometimes eats a 95% vegetarian diet (gelatin being the 5%) and she frequently gets in over 100g each day while eating a low fat diet. Could go the other way and eat a low carb diet, I've done this before but kind of got sick of eggs all the time.
Check out the protein content in stuff like cottage cheese.
