A couple of interesting studies re: email use and our responses to it, via Jeremy Dean of PsyBlog.

The Impact of Capitalization and Emoticons on Perceptions of Email
Apparently, depending on your personality type, proper use of capitalization and the use of the smiley emoticon can make you seem more likable. According to one study cited by Dean (Byron and Baldridge, 2007):

They found that, sure enough, using correct capitalisation and emoticons tended to make a better impression on readers. The reader's personality also influenced how emoticons and capitalisation were perceived. Readers high in both extroversion and emotional stability were likely to rate sender's emails as more likeable if they had correct capitalisation. As for emoticons, readers higher in emotional stability were likely to rate sender's emails more likeable if they used emoticons.

The opposite was also true. This meant that for the introverted and emotionally unstable, correct capitalisation tended not to affect the sender's likeability, perhaps even lowering it. Similarly, emoticons had little effect on the emotionally unstable.

Communicating Persuasively--Email or Face-to-Face?
Another study indicates that email tends to work better in persuading men than it does women:

Persuasion research has uncovered fascinating effects: that men seem more responsive to email because it bypasses their competitive tendencies (Guadagno & Cialdini, 2002). Women, however, may respond better in face-to-face encounters because they are more 'relationship-minded'. But is this finding just a gender stereotype?

While this appears to be related to gender, there may be more value in looking at the nature of the relationship, says Dean. If you have a competitive relationship with a person, then you might be better off using email to make your case. If your relationship is more cooperative, then face-to-face may be the better way to communicate.