Empty Words _ Since most personal statements are about 5μ lines, you don't have space for empty statements such as 'I am a hard worker' or 'I am a people person'. How can the hiring manager ascertain that this is the truth without any evidence to back up such lofty statements? You need to give them examples of your work ethic and how it positively impacted your employer as well as evidence of your interpersonal skills.
Keep It Readable _ You will also want to keep your personal statement/essay to a readable length and writing style. Trying to use large words and convoluted sentences to make the personal sound more impressive will only lead the reader to put the paper down.