According to Brown (2007, pp. 326-327) notes additional features of the spoken language that make a big difference between writing and speaking and that they all are really hard for English students to learn.
CLUSTERING: Fluent speakers group words together rather than uttering each word.
This consists of phonological processes and Stress patterns implied into the language. For example: This happens in Referential Stress (Numbers, affixes, etc.), Differential Stress (Noun/Verb & Adjective/Verb), Differential Stress (Noun Compound), Phonological Processes: Assimilation (Progressive, Regressive, Coalescent, and Gemination), Dissimilation (Haplology) and Linking processes.
REDUCED FORMS: Speakers use contractions, elisions, and reduced vowels. For example: This tends to happen when a phonological process called Deletion appears.
PERFORMANCE VARIABLES: The speaker is permitted to pause and hesitate, using filler words and expressions such as uh, you know, like…
This supplies a more natural speech for users to sound more like a native speaker.
COLLOQUIAL SPEECH: Informal terms are permitted and common.
Idioms, Slang and informal expressions are COOL.
RATE OF DELIVERY: The speaker controls the rate of delivery for the listener; a reader can read at his or her own pace.
STRESS, RHYTHM, AND INTONATION: Rhythmic and intonation patterns can be important in conveying meaning in spoken language.
Eduardo Becerrit 