Chat speak can be invaluable, as your rightly note, for conveying tone in e-mails. It can turn what would otherwise seem mean into a friendly jab. Three things that do annoy me. 1) the use of chat speak in lieu of a standard English expression that better captures the point or at least captures it just as well. 2) The use of chat speak as a kind of speech tick. I have received e-mails where "lol" was used in ways that just looked like e-mail nervousness (like a person giggling inappropriately because they are stressed). One of the benefits of e-mail is that it allows you to collect yourself and not be nervous in your communications with others. 3) I have students who seem to have lost the ability to write a formal e-mail. E-mails to your professor aren't simply text messages done on a computer. This is especially so when you are asking for a favor that will take a substantial amount of time to fulfill. Admittedly this isn't the fault of chat speak per se, but excessive chat speak does cut down on the formality of communications, and some communications should be formal in tone.