The turnaround time can be faster. So email is more conversational.
In a paper document it's essential to make everything unambiguous because recipients not have a chance to ask for clarification. But with email documents, your recipient can ask questions immediately.
Never you need to be so formal in an email to tell your co-worker that you are ready to go to lunch, and you never wouldn't do that on a paper letter.
Your correspondent also won't have normal status cues (such as dress or diction). So may make assumptions based on your name, address, and facility with written language.
Email also does not convey emotions nearly as well as face-to-face or even telephone conversations. It lacks vocal inflection, gestures, and a shared environment.
Your recipient often have difficulty telling if you are serious or kidding, happy or sad, frustrated or euphoric. Sarcasm is particularly dangerous to use in email.
Another difference is that what the sender sees when composing an email might not look like what the reader sees. Your words tone only sound in your mind.
Your message's visual style may be quite different by the time it gets to someone else's screen. The software and hardware that you use for your emails may be different from what your recipient uses.