A whole range of ’testamentary intentions’ have been found to be valid wills in the right circumstances. In very broad terms it is a matter of having sufficient capacity to make a will (which is presumed in the absence of evidence to the contrary), sufficiently identifying who is making the will, and recording their intentions for their estate.
Mistakes don’t stop a document from being a will, but they do potentially cloud the testator’s intentions, give rise to alternate interpretations, and invite doubt - all of which are exactly what you want to avoid with a will.
Wills can range from the ‘very simple’ to the ‘incredibly complex’ from the perspective of a trained lawyer. To an untrained lawyer it may appear to be a simple job, but it can be a minefield. The importance of crossing t’s and dotting i’s should not be undervalued. A simple comma in the wrong place can potentially be disastrous, even if it ‘simply’ causes a long-winded (i.e. expensive and time consuming) court proceeding between beneficiaries and a trustee with the ultimate effect of eroding the estate. Because, ultimately, when a will becomes truly contentious is when the person who made it is no longer available to explain their intentions. All you have is what’s in the will, and if that is unclear then it will be down to a court to make a decision about what it means.