1. Not enough leading or line spacing.
Line spacing helps improve the readability of huge blocks of text on a page. Readers can easily follow lines of the text without losing pace. So when you decide on the spacing of your text remember to use optimal spacing depending on the different font types and sizes. Too little space make it cramped and impossible to read. Whereas huge line spaces make it different for the eyes to easily track the flow of the text.
2. Not enough tracking
Tracking, otherwise known as letter-spacing is usually applied to a group of letters. It helps improve the readability of the text by preventing letters from running into each other. Appropriate spacing between the letters help clearly decipher them thereby improving readability.
3. Proper kerning
Kerning deals with the spacing between two letter pairs. Kerning should not be confused with tracking as tracking is the space applied to a group of words. Kerning works well when applied to headlines, texts which are all in capitals and logos.It helps improve readability with various sizes.
Also while dealing with spacing in general, don't space everything out such that it becomes illegible. For example : D I F F E R E N C E B E T W E E N T R A C K I N G A N D K E R N I N G . ( Difference between tracking and kerning.)
4. Lengthy lines of text can be boring and confusing to follow.
Making text a huge continuous block is not at all inviting, it makes the text boring and difficult to read. Keep your lines short and easy to read. Short sentences are easier to understand and it will help you convey the message more effectively.
5. Mixing too many typefaces and weights
This could make your site look haphazard and uneasy to follow.Try and stick to a maximum of 3 font choices or lesser. Too many weights tend to confuse the reader as they are unable to discern the important elements on a page.
6. Using sans serif font for lengthy text
Serifs are known to work better when it comes to putting up a lengthy reading material as they render less strain on the eyes. The feet or serifs as they are called of each letter clearly outlines its shape and make it easier to define the letter. So while reading lengthy text like the body of any project,they are easy on the eyes.
7. Using shades that do not contrast well.
 The above illustration explains why it's a bad idea to use colors of similar value one on top of the other.
8. Reversed out text on less than 50% tints
Just as the above example this point is also better illustrated through the image. Reduced tints make it difficult to define letters and so make it difficult to read such texts. Such texts make prints which are far less visible than that which is seen on the screen.
9. Centered text – a big no…
Formatting paragraphs a important, but when you do so choose the appropriate alignment. A center aligned para is a big no as its sharp and jagged ends make them unprofessional.
10. Not knowing what the Grid System is
The grid is one of the best tools or format for layout of the text on your website. It gives better clarity and makes your fonts and layouts more cohesive.