Usability testing includes the following basic standards
1. Colors used applied to the text and screens
2. Font size
3. Information text given to user or validation messages
4. Types of icon used in the application
5. Type of language used should be understood easily
and the above differs from application to application.
so, target the types of users for your application and idenfiy the issues based on your previous experience or compare the look and feel of the any similar kind of existing product.
So you want an answer that is more vague than this one that has been provided? (It is a good answer by the way)
Something else to consider is whether you have a requirement for use by users with accessibility issues - For example is the text in the HTML (I am referencing web for this example but there are others) formatted corectly to be interpreted by test to speach conversion programs?