The most common failing students have in their writing is the lack of development, or of detail. "Development" means that you do not just make a claim, but instead you support it with detailed examples and factual explanations. It means that you add specific, exact, and thorough information about your point. The more specific you become, the more details you give, the more you develop your paragraph.
One common failing is that a student will simply repeat the same information again and again, just using different words:
Traffic in Tokyo can be dangerous to a driver who is not cautious enough. If you are not careful, the various vehicles on the streets of Japan’s capital city can cause you to fear for your life. For example, motorists often take risks which will create a threat if you are not alert. In the streets of Tokyo, there are just too many opportunities to meet a quick death due to lack of attention. Furthermore, if a driver does not watch out, people in other automobiles could cause such a hazard that you will never want to drive again.
The paragraph above simply repeats the idea: [Traffic] [in Tokyo] [is dangerous] [if you are not careful]. The idea is not developed because no specifics are given. Why is Tokyo different from any other city? What are specific threats? What will happen if you are not careful? You have to explain about how taxis suddenly swerve and stop to pick up a customer, or how scooters dangerously zig-zag through traffic, or how streets are so narrow that you will not see the woman with the baby stroller until it is too late.
More information and examples are given in the chapter on minor supporting details.
The other major failing students suffer is the simple lack of specific words. Your writing should express such detail that the reader can see what you are writing. Don't just mention that your younger brother's room is "very messy"; describe the horror! Write about how so much trash covers the floor, you cannot enter the room without stepping on sweaty old underwear; crunchy old pieces of stale, greenish bread; or something that feels cold, slimy, and wet in your toes, but you don't know exactly what it is. Give details!