Simple, Ordinary Language
You don't need fancy language to make your point in writing. Jargon, acronyms, and "big words" can be confusing to a reader, and can obscure the underlying point. Effective writing uses simple, common words and simple grammatical structures. It uses concrete words rather than abstract ones. Good writing avoids the passive voice (which means this sentence should never be written: "The passive voice should be avoided in good writing").
A Clear Purpose
Most written communication exists to make a point or to tell a story. Effective communication makes the central point of the message or story clear. No one reading a well-written piece, be it an email or a report, should be confused about the purpose behind the writing. Persuasive writing explains the goal of the writer clearly and unambiguously, asks the reader to do something specific and explains why the reader should respond as the writer requests.
A Positive Attitude
Effective communication tends to be positive in tone, and shouldn't be rooted in anger. It may feel good to write an email or an office memo while you're angry, but it's not likely that the result will be effective at producing the results you want.
Effective written communication is brief and to the point. Every sentence is meaningful. Every paragraph carries one major point and makes that point well. Longer isn't better when it comes to writing, because overwriting doesn't help your message. Overly long sentences can be hard for a reader to unravel and understand. Overly long paragraphs can be daunting for a reader, who may choose to put the communication down if he feels intimidated by its length.
A Conversational Tone
Effective written communication uses a conversational tone, and never sounds formal or stuffy. It uses active verbs to help the communication feel lively. Sometimes striking a conversational tone means using contractions ("I'm," "you're," "don't," "can't"). It can even mean writing in the first person ("Here is what I just learned"), though this isn't appropriate for all forms of writing, particularly reports and news articles. Especially effective writing finds a point of connection between the writer and the reader, which will make the reader want to keep reading.
Professional Grammar, Spelling and Punctuation
Effective written communication is always punctuated and spelled correctly. Sloppy grammar, spelling and punctuation shows disrespect to the reader and reflects poorly on the writer. It's particularly important to spell names correctly when writing direct communications such as letters and emails, as some readers are deeply offended by sloppy writing. Spell checkers can catch a lot of errors, but it's crucial to proofread personally as well to catch what an automated program might miss.