Off the top of my head . . . Just about impossible to get sufficient vitamin B12. Difficult to get all essential amino acids. Vegetarian sources of omega-3s are not as easily assimilated as animal sources.

You just need to add supplements to it same as any other diet.

Spirulina
veggie DHA/EPA
l-lysine
l-taurine

Other than those and maybe a couple of other supplements you are much better off on this diet. I am about to about 60% raw and will hopefully be at 75% by mid summer. :)

Here are a couple of articles on the subject:
http://rawvegans.tribe.net/thread/5fe4a04d-2de4-44e1-bf12-704a78f5ef46

This one is from beyondveg.com, which has much more info in addition:

http://www.beyondveg.com/nicholson-w/hb/hb-interview1a.shtml

This is good too, though not quite on the subject:

http://www.beyondveg.com/bratman-s/hfj/hf-junkie-1a.shtml

I've been doing only raw food (fruits, vegetables, nuts) for about a month now. The worst thing is not having enough iron.

The human body is actually better equipped to digest raw meat than raw vegetables. Not everything should be eaten raw; many plants contain irritating substances that are neutralized by cooking (oxalic acid, etc.)

A previous poster mentioned B12--the analogs of B12 in spirulina, chlorella, and other plant foods are NOT usable in the human body as B12. Other essential nutrients vegans may lack are vitamin A (true vitamin A, or retinol, is ONLY available from animal foods. Beta carotene isn't vitamin A, and many people can't effectively make the conversion from carotenes to vitamin A) and EPA/DHA from fish. I fear for children who grow up in vegan households.

It can be hard to make sure you're getting the right mix of amino acids as plant proteins aren't complete, and you really need to know what you're doing. Soy is a complete protein, but is very low in two amino acids and is toxic to humans unless fermented for several years.

My biggest concern with a vegan diet would be replacing meat with soy products. Some people base their entire diets around soy! Would we base our entire diet around chicken? Or carrots? Or lima beans?

I won't waste a lot of space venting about soy, but I'll provide some good links. I also highly recommend Dr. Kaalya Daniels' book The Whole Soy Story as an excellent, well-referenced and thorough investigation into this "miracle" food.

http://www.westonaprice.org/soy/onewoman.html

http://www.westonaprice.org/mythstruths/mtvegetarianism.html

http://www.second-opinions.co.uk/vegetarian.html
(This guy is a little extreme but has some good Science on his site)

http://www.second-opinions.co.uk/miami_death.html

http://www.thewholesoystory.com/index.php

Wonder what the backlash will be from this one...oh well. This is a debate forum after all.