Most common deficiencies in vegans and vegetarians are B12, iron and sometimes protein. While iron and proteins are fairly easy to obtain through many foods like various nuts, beans and whatnot, B12 is rather tricky. The only meat-free food that contains some noteworthy amount of B12 is eggs and some cheeses, so if you're vegan and don't even eat these it's pretty much over for you and you have to take supplements or suffer.
I've been a vegetarian for more than 15 years, mostly didn't have any problems, never considered veganism, but I also started eating some meat and fish here and there in the past years. After eating some very tasty Lahmacun recently I have made the plan to eat meat once a week.