Is it just me? Whenever I see "rotfl" or "lmao" pop up in the middle of a serious discussion, intended to convey how stupid the opponent's point was, I have an almost visceral reaction. I immediately begin to view the author as either borderline retarded or afflicted with anti-social personality disorder, and lose all interest in anything he or she might have to say...ever again.
If you don't share my reaction, that probably sounds unduly harsh, but let's think about the context for a moment. A serious discussion is in progress, perhaps about the legalization of prostitution or Sarah Palin's qualifications for the Vice-Presidency. Both "sides" seem to have strongly held views and have thought the issue through. Then, one makes a point...and the other writes "lmao".
So there are two possibilities, right?
The first is that the person truly IS laughing, having lost all sight of the actual issue on the table because she saw an opportunity to mock someone and dropped the serious shit like a hot potato.
The other is that she's NOT laughing, but has voluntarily departed from intelligent debate because it's more important to her to try to make someone else feel stupid than it is to convey a valid point.
Neither of those things gives a writer a lot of credibility in my book. It's rather like a child yelling, "I know you are, but what am I?" at someone who has said, "Give that toy back to Kevin" or "It's time to do your homework": a spotlight on all that is weak, childish, defensive, and even incoherent about the speaker.