Alignment

When creating blocks of text, stick to left-aligned (Flush left, ragged-right).
Justified alignment should only be used in a formal setting: contracts, bank prospectuses and the like. (It's frequently used in books and magazines to maximize space)

Also, full-justification is difficult to read by someone with a learning disability such as dyslexia.
And, we designers hate the gaps and rivers it creates.

Click Here for a great book on design basics such as alignment.