Law makes will-making easy
Even back in law school, it seemed odd that the law in most countries would allow people to pass on their all-important personal assets (savings of a life-time) by a scrible on the back of a beermat, on a post-it or in a simple letter. Handwritten wills (properly written out, signed & dated) are considered valid in most jurisdictions. Such is the law, but you can imagine it has lead to many disputes and court cases.
French law was the common source: under Napoleon, the custom of handwritten wills became law and spread to almost every civil law jurisdiction in Europe, and half of states in the United States.
The simplicity of handwritten wills (also called "holograph" or "holographic" wills) makes them an accessible instrument to anyone who can read and write. even being able to read is not really a prerequisite, as even blind people have been known to make valid written wills. Nor is writing by hand a strict requirement, as wills wriiten by foot (or mouth) have also been recognised as valid ... as long as the will proves to be authentic. Authenticity of handwriting or signatures can indeed be established by expert analysis in most cases, as opposed to the authenticity of typewritten texts or electronic signatures (easier to copy or forge).
A will written in the snow, or with a digital pen on a computer screen, may be considered as valid but case law to back up this theory is scarce, so it is best to stick to old methods (pen and paper).
On 8 June 1948 in Saskatchewan, Canada, a farmer named Cecil George Harris who had become trapped under his own tractor carved a will into the tractor's fender. It read, "In case I die in this mess I leave all to the wife. Cecil Geo. Harris." The fender was probated and stood as his will. The fender is currently on display at the law library of the Univalersity of Saskatchewan College of Law.
Testamentary intent is of course crucial (expression of testator's definite intention). The written document, if not referred to as a last will (testament) by the person writing it, should carry at least a reference to death being the event triggering its enforcement. Or it must be otherwise clear from the content of the document that it is a last will, and not just an expression of a current wish or intention.
If making a will is easy, it is more complicated to give effect to a last will (e.g. through probate or letters of administration in Common law countries, e.g. through notarial or court intervention in Civil law countries).
Whenever wills are contested, long court proceedings can cause serious delays in the enforcement of the will. Delays can be avoided by writing the will in clear wordings and document as much as possible the making of the written will (leave traces, confide in people who can testify about the existence of the will...). Especially when the testator has lived in different countries during his life, his or her handwritten will - even when registered - risks not being found.
Registration of wills does exist in most countries, but the national registries are not yet (or just to a limited degree) interconnected. Information on Will Registries can be found here.