1. Having too many lists - Many people have more than 1 list for different roles in their lives and this can be quite confusing. It is best to have 1 list that you use each day to make things much more simpler.
2. Not simple enough - Some people have complex lists that are too confusing and not simple enough. Just write out what you need to be doing and keep it as your list.
3. Too many tasks on the list - I try to keep my task on my list to about 6. Some people have too many tasks that it can be overwhelming and can cause you to feel down when each day you fail to finish all the tasks for that day.
4. Not visible - Your to do list should be visible at all times when you need to see it. This is the advantage of paper lists than electronic ones. Make sure you can see it whether you are home, in the grocery or even driving.
5. Not using it - This is obvious but if you have all those things mentioned above and you don't ever use the list then you are just wasting time writing things down. Remember there is a difference between activity and productivity.