A bad or poor letter of recommendation
1. Is neutral. Glowing letters of recommendation are the norm. A neutral letter is the kiss of death to a student's application. If you can't write glowingly positive letter, don't agree to write on a student's behalf because your letter will hurt more than help.
2. Has errors, such as typos and grammar mistakes. Errors suggest carelessness. How good a student is this if you're not willing to run his or her letter through a spell-check?
3. Discusses weaknesses without discussing strengths. If a student has an important weakness, you ca mention it, but remember to discuss many many strengths to balance it out.
4. Provides no examples or data to support statements. Why should the reader believe that a student is meticulous, for example, if you haven't given an example to explain how?
5. Shows that the letter writer has little experience and contact with the student. Don't write letters for students you don't know. They will not be helpful letters.
6. Is not based on relevant academic or applied experiences. A letter for a student that you have had no academic or supervisor experience with will not help his or her application. Don't write for students who are friends or family members.
7. Is late. Sometimes incomplete applications are tossed after the deadline. Even the most fantastic letter will be of no help then.