Becoming a Vegetarian is a personal choice that can be a bit confusing. Many people are able to do it successfully love it, others may be doing more harm than good. Correct nutrition is a very controversial subject but let's look at some major drawbacks of going veg. 
How to do it wrong:

Just omitting animal consumption. Vegetarianism needs to be about more than just omission. Those who want to take on vegetarianism should be thinking about all the new sources of nutrients that they will be adding into their diet. Make a list of grains, beans, legumes, vegetables, etc.. go shopping and plan ahead. Making sure your nutrition plan is complete may take some planning ahead. Discovering new food sources will be fun and exciting!

Using dairy for all protein needs: problem. Most dairy that is available to us these days contains hormones and antibiotics. In small doses it may not create a problem, but for those consuming milk cheese & yogurt several times a day as a main source of protein...it might become a problem, most likely to show up later down the line. Not to mention that many people are lactose intolerant..meaning gas, bloating, and other unpleasant side effects. Hint: your tummy shouldn't be upset every single day. Sounds obvious, but I've met people who didn't know the difference.

Not using supplements: Supplements are not necessary for everyone, but for vegetarians who are more susceptible to deficiencies, supplements can help. I recommend doing your research, reading about common deficiencies among vegetarians and then making a well informed decision about what to include.
Listen. No matter what you eat or don't eat someone is always going to find a problem with it! I say just enjoy your life, find the best nutrition plan for you that makes you happy and produces the results you want! Life is short, live it how you want to and not by anybody else's words! Cheers to your happy belly, gorgeous body and healthy mind!