@Purple:
The killing joke provided a narrative told from the Joker's view so you could get a sense of how it was to think from inside the character. It's not the greatest backstory ever, and they even prod that idea with the Joker himself saying that he doesn't even know if that's true anymore. The backstory was a complement to seeing inside his mind but it doesn't exactly define the character in any way.
I honestly thought that the Luffy flashback was incredibly weak and didn't provide us much of any insight that we didn't already know about him. It'll be important down the road with the heavenly dragon folk and Sabo's origins as part of the rich folk, but Luffy is, well, still Luffy after all that. It doesn't really give me a bigger picture as to Luffy's wildness or hunting for food or whatever. It certainly doesn't really make me feel any more emotionally attached to Luffy than I already was and in the end we already got most of the taste of his development through small quips like Garp throwing him into the woods tied to a balloon way back in Water 7. Either way, even if that flashback wasn't there, nobody would be asking "why does Luffy never think things through?"
You can have great backstories that go in line with the plot that also shed light on characters. Chopper's is great emotional pull and also explains the main beef against Wapol. Nami's tells where she comes from and her position against Arlong. Alternatively you have really weak back stories like Zoro and Usopp (hm, wasn't this discussion in the Naruto thread?), but regardless of the lack of any real depth to their flashbacks, they're still very colorful characters that add to the cast in a very dynamic manner because of their development throughout the series.
I wouldn't call Usopp's backstory weak. It was just simple compared to the rest but was pretty endearing and explained alot about him.
The villagers thinking about it and why he always yelled the pirates are coming was the perfect compliment to it.
Now that I think about it isn't Yasopp just as bad of a father as Ging?