Answer:
If you are an employer, decide what factors to include on an "Employee Evaluation". Then, base your comments on whether the employee satisfied each factor. Use a neutral tone in statements, focused on performance, attitude, promptness, thoroughness, etc. If there are problem areas, be direct and clear but also state how the employee can improve. Let great employees know how they contribute.
If you are an employee and asked to comment on your employer or workplace, think about your company's goals, and your job description. Keep comments neutral, rather than too personal. State what works well, what is lacking, and how the company can help employees through specific changes.
Whether employer or employee, keep personal feelings out of evaluations. Be polite, calm, give specific examples of good or sub-par performance.