From the synopsis my friends gave last time: GW1 = CHAR WAN 2 KILL DE HUMANZ. GW2 = CHAR FRENZ WIT HUMANZ, BUT SUM HUMANZ WAN 2 KILL DE CHARZZARD.
Obviously, a gigantic generalization. But yeah. Definitely right that knowing the backstory isn't really necessary for MMOs. You watch the cinematics, read the quests, and you'll pick up the story as you go. A lot of things from previous games will be hinted at, but it won't hurt your gameplay experience at all if you don't read the wiki first. You may just miss things like "AH HOLY SHIT IT'S THAT ONE GUY WHO LOST HIS EYE IN THAT ONE QUEST IN GW1." And he now has an eyepatch.