More seriously......
On GSA, I will usually chat after the game about what each of us did right or wrong, and how our decisions may have made the game end differently. Hopefully I don't sound too much like an arse when I mention the mistakes I made that may have let my opponent get in a good volley or two.
On D2, I'll usually just say "GG" or "Well Played!" if they blow me up (so long as the session doesn't end before I can type it out) unless they hail me in general chat and want to discuss things. I try to not spend much time in general chat, however, as the game keeps going while I type. So usually the conversations on D2 are much shorter than they would be on GSA.
Sometimes I have to leave a PvP before a fight is over. A usual departing line from a plasma vs. plasma battle that ends that way might be, "We could probably play for another hour, but I think the damage done so far gives you the win and I've got to go."
-S'Cipio