Most vegetables are edible raw, but some are more palatable than others. You should be cautious about just a few vegetables when consuming them raw as they can have toxic elements or interfere with digestion.
Yucca, also known as cassava root or tapioca, is commonly consumed in Latin America -- but only after being dried, soaked in water, rinsed and cooked. The leaves of yucca contain cyanide, which makes them inedible to humans, animals and insects. The starchy root also contains some cyanide, which is removed with the soaking and cooking process. Rhubarb stems, popular in pies, are perfectly OK to eat raw. The leaves, though, contain significant amounts of oxalic acid, which can cause kidney damage and even death if eaten in large quantities. Sweet potatoes are perfectly healthy to eat raw, but consume white potatoes raw only in moderation. Potatoes with "eyes" may contain toxic alkaloids. Raw potatoes also contain anti-nutrient compounds that inhibit your body's ability to produce digestive enzymes, which are essential to nutrient absorption. You may also find that raw potatoes cause digestive distress, including bloating and gas.
Raw dieters espouse the nutritional benefits of consuming raw veggies over cooked ones but miss that certain nutrients can actually be unlocked with heat. Cooked tomatoes are a major source of lycopene, an essential antioxidant linked to lower rates of heart disease and cancer. "Scientific American" also reports that cooked carrots, spinach, mushrooms, asparagus, cabbage and peppers are among the vegetables that actually offer more of certain types of nutrients, including carotenoids and ferulic acid, when steamed or boiled. Cooked veggies do lose significant amounts of vitamin C, however.