Walnuts are one of a kind, highly and nutritious. In comparison with other nuts, they are lower in saturated fats and rich in omega-3 fatty acids. "Compared to most other nuts, which contain monounsaturated fatty acids, walnuts are unique because they are rich in n-6 (linoleate) and n-3 (linolenate) polyunsaturated fatty acids. Walnuts contain multiple health-beneficial components, such as having a low lysine: arginine ratio and high levels of arginine, folate, fiber, tannins, and polyphenols. Though walnuts are energy rich, clinical dietary intervention studies show that walnut consumption does not cause a net gain in body weight when eaten as a replacement food. " This makes them pretty much the only nut of choice for people with cardiovascular disease, when eaten in moderation.  In contrast, cashews, almonds are toxic when eaten raw and even upon steaming, they can be highly allergenic as well (alfa toxin, amygdalin -almonds, urushiol -cashews). Urushiol is a highly allergenic oil, found also in poison ivy.
Walnuts are very rich in vitamin E, and studies have shown that vitamin E from diet cuts the risk of Alzheimer's by 25%. Alzheimer’s disease (AD) is a severe neuro-degenerative disease that gradually results in loss of memory and impairment of cognitive functions in the elderly. The aggregation and fibrillization of amyloid beta-protein (Aß), leading to the deposition of amyloid plaques in the brain, is one of the major pathological features in Alzheimer disease. Besides this, walnuts contain other beneficial substances for Alzheimer. "Amyloid beta-protein (Aβ) is the major component of senile plaques and cerebrovascular amyloid deposits in individuals with Alzheimer’s disease. Aβ is known to increase free radical production in neuronal cells, leading to oxidative stress and cell death. Recently, considerable attention has been focused on dietary antioxidants that are able to scavenge reactive oxygen species (ROS), thereby offering protection against oxidative stress. Walnuts are rich in components that have anti-oxidant and anti-inflammatory properties. The inhibition of in vitro fibrillization of synthetic Aβ, and solubilization of preformed fibrillar Aβ by walnut extract was previously reported."
Even more, walnuts were proven to be beneficial for cancer patients. "A high -fat diet containing whole walnuts (juglans regia) reduces tumour size and growth along with plasma insulin-like growth factor 1 in transgenic adenocarcinoma of the mouse prostatate model". 
This food is very low in Cholesterol and Sodium. It is also a good source of Copper, and a very good source of Manganese. Offers 4.3 g almost complete protein (lysine is lower) per 28 g walnuts. And 2542 mg omega-3 fatty acids per 28 g.
Interestingly this food looks like a brain and is very good for brain health. When carbohydrate metabolism is affected in brain cells, like in many neuro-degenerative diseases, brain cancer, ADHD, the alternative fatty acid metabolism is the best metabolic pathway to go. In this case walnuts are highly beneficial as they are 83% fat (mostly fatty acids and monounsaturated fats), 8% protein. As we know, Alzheimer disease is correlated with cardiovascular risk as well, so how come walnuts are good? well walnuts in particular were found to improve the lipid profile of hyperlipidemia patients in a clinical study. LDL was lowered and HDL increased, a decrease in apolipoprotein B levels was also observed.