Disappointment is dissatisfaction, caused by an act or a happening that is lower than expectation. Disappointment has many faces; it could be of any nature or type. A letter of disappointment can be written in personal capacity (friends, family, colleagues, relatives, social circle) that is informal letter, in case where there is a disappointment by the act of any person, or someone let you down. On the contrary the other type of letter of disappointment is the official or formal, that entertains the dissatisfaction of a company with another, customer disappointment, complaint of an organization towards worker, complaint towards government, disappointment over standard of a product or quality of a work.
In both cases there should be a ground or a valid reason behind the letter of disappointment, you must be sure and clear that what has happened to you who is responsible for that, a point of consideration should mould whether you have demanded the thing or work you are complaining for. So first of all your letter should be factual, as it will not be considered if it does not have a ground based on facts. A letter of disappointment should be authoritative; it means it should be presented professionally, and should be having credibility. This feature becomes more important when it comes to formal letters of disappointment.
No matter what we are complaining about or what has pushed us to write a letter of disappointment but it should be concise, well written and comprehensive. You should not sound rude instead the tone of the letter should be friendly. Your point should be conveyed in a manner that will help to take the issue towards a settlement so the letter should be having a co-operative and complementary tone. A well written letter of disappointment should be having some suggestions which will lead to a situation instead of just complaining about the things, such letters are not considered, not only suggest way out but you can also demand a compensation.