When you meet up with friends (be it in RL or just chatting via IM) you often talk about things you have done in game: quests, raids or PVP. For example you can say that you've just finished "Northern" quest line and that it had good story and involving objectives. Your friend may nod and agree it was fun.. that's it. Quest was identical for both of you so there is nothing to talk about. Of course, there is this one exception that things might have went wrong and for example you can tell the story of how your partner died and you barely finished the last quest alone by killing some big boss while you had only 5% hp left. Basically in your standard MMO, your accomplishments (quests) don't hold any bragging or story telling value until things go very wrong. Because content is fully static, your friend knows EXACTLY what happened otherwise.
Things are quite different in case of GW2. When your friend tells how he cleared "Eagle's Fort" area from centaurs patrols/scout parties, you can counter that with your own version. Instead of few scouts, you've encountered full fledged assault on the fort, you and your friends tried to hold it. The longer you've hold, the more centaurs would come. You've started to target their lieutenants and supply lines to even out the chances, but that only provoked centaur's commander who personally joined the fight. You were struggling so you've called reinforcements, more of your guildm8s arrived. This enraged commander even more unlocking more of his abilities resulting in one epic boss fight in the middle of half ruined fort which you are trying to defend. The best part is your friend still have no idea if you succeeded or failed to defend it! But by now the out come doesn't matter any more, you've already have one jealous friend listening to you with wide opened eyes and wishing "he was there" to see it.
That's the beauty of dynamic event system, you have actual stories to tell, real adventures. I don't think people realize how much depth it adds. Of course not every DE you encounter will develop so gloriously and that's good, that makes the "I wish I was there" moment even more important. Some may see it as a "bad" thing cause it will naturally lead to some people "missing out on stuff". Overall though I think it's fair price for creating unique character development experience.