How many memorable characters it has. When you see the ninth (or tenth) opening in the anime that uses that remix of "We Are", it becomes amazing how many of the characters you can recognize. When the main characters encounter old characters, your reaction tends to be similar to that of the main characters.

Also, there's how Luffy's stretching is handled like a combination of balloon and those stretchy hand things that you can get out of capsule machines for a quarter instead of the way that just about every stretchy super power gets handled. And the way it's handled with Luffy actually ends up seeming realistic in comparison.

And then there's the wide variety of animals. Just look at the list of them at http://onepiece.wikia.com/wiki/Animal_Species. It is amazing how long the list is. It might be possible to make an entertaining anime that involves some guy in khaki shorts and an Australian accent going around the Grand Line and teaching us about the sea kings, hiking bears, kung-fu dugongs, south birds, and other interesting creatures while showing about as much fear towards the most dangerous animals as Luffy would. And now if you'll excuse me, I need to find out if there is any Crocodile Hunter and One Piece crossover fanfiction.

Looks like it's time for a new word for the plebeian masses.

Flibbertigibbet

And with that, this flibbertigibbet is ready to go back to sleep and dream of creative ways to use a marshmallow shooter, instant snow, a rubberband ball, and one very irate penguin to take over the world.

And for those who wonder how I discovered the existence of such a word, I would like you to visit this list.

I just learned that penguins really aren't jerks that would knock other penguins into freezing cold water.

I also learned that the digits of Pi can make a rather catchy song.

@TakinawaTonfa:

So is "plebeian" technically the new Word of the Day or can I throw "shibboleth" out there?

Correct on the first half. Just about any plebeian eye should be able notice that.

I have learned that there are people who are unable to tell the difference between the words "all" and "most". The same person was also having trouble telling the difference between "some" and "none". I learned this due to my interest in seeing idiots who think they can argue against something they know nothing about being told just how wrong they are.

For those who weren't paying attention, "callipygian" is an adjective, not a noun. I would think that the proper use of adjectives was a plebian skill that any of the common men would be capable of.

I learned that someone thinks that gamers need their own cookbook.
http://gamerscookbook.com/

I also learned how to make a metal, glowing cube that I could show to people to make them think that I came from the future.
http://www.instructables.com/id/Awesome-led-cube/

I also learned that there is at least ONE alarm clock that doesn't seem like an evil being intent on ruining my day by waking me up in the most annoying way possible. Just listen to the mp3s of it and tell me that you wouldn't want to wake up to that voice.
http://www.thinkgeek.com/homeoffice/lights/aef3/

Looks like enough time has passed for a new word.

Callipygian
http://www.merriam-webster.com/dictionary/callipygian
http://dictionary.reference.com/browse/callipygian

I wouldn't mind seeing some girls showing off just how callipygian they are.

Sorry to have joined the discussion rather late, but I have something to say.

First, I have been noticing a bit of a pattern for the characters that join the crew. Each of them are given detailed backstories that seem to be trying to be sadder than the last. Just look at the list below.

1. Luffy (got kidnapped and almost killed and his idol lost an arm to save him)
2. Zoro (best friend died)
3. Usopp (mother died of illness)
4. Sanji (got separated from everyone he knew (for all he knew, they could be dead), almost starved to death on an island devoid of any life except for him and one pirate)
5. Nami (parents died, adoptive mother murdered, spent years working for the people who murdered her adoptive mother)
6. Chopper (everyone hated him for being different (including his parents it would seem), poisoned only friend while thinking the poison was medicine (but the murder being prevented by a suicide probably softened the blow from that))
7. Franky (ships he made were used to frame his father figure, got most of his body destroyed while trying to save father figure from being executed)
8. Robin (early life resembling that of Cinderella or Harry Potter, most hated her for having a DF power, only friends formed a group that they wouldn't let her be a part of (although it was for her safety), has to be seperated from her mother as soon as she is reunited with her, has to run away from her home, has to spend the rest of her life on the run from the World Government and relying on the more evil pirates for protection, and did I mention the island that was her home was destroyed along with everyone else who lived on it?)
9. Brook (has just about everything that has any meaning to him taken away, saw everyone in his crew die one by one until it was just him left, died himself, spent about 50 years alone at sea alone, eventually loses his shadow and the ability to survive in sunlight, sees his techniques being used by someone who possessed none of the morals that are supposed to be taught alongside the technique, almost loses the last physical trait that would allow the person he made a promise to to recognize him, and he has to ask someone else to do the work of recovering his shadow)

Keep in mind that the above list is in order of when the backstories are told, not when the person joins.

If I see a backstory that is more than a few images with a description of the backstory being given by one character to another and I start to think that it rivals those of Robin and Brook, then I'll be convinced that the character will join the crew.

As for right now though, Boa's backstory just isn't detailed or sad enough, but it does seem possible for it to be expanded into one that would be. However, I do see her as a good candidate for being a temporary nakama (like Vivi was), and I think she may end up fulfilling the same role in Luffy getting a bounty increase that Nami, Vivi, and Robin have.

As for Jimbei, we don't quite know much about his backstory, just that he had a pirate crew, became a Shichibukai, and recently lost his Shichibukai status and was taken to Impel Down. However, the end of chapter 529 suggests that we will get to see his backstory.

Perona, Mr. 3, and Mr. 2… we know absolutely nothing about their past with the exception of some mere speculation on Mr. 2's birthplace.

Before I respond, I feel the need to get something off of my chest. For years, I've been told that I have a mental disorder. I've been told that I had ADD for most of my earl life, and more recently (about 5 or so years ago), I've been diagnosed with Aspergers (yeah, that one who's symptoms seems like a description of your average geek). I will say right now though that despite what any psychologist has said about me, I am not mentally handicapped. Sure, I may be a bit obsessive about one or two things. Sure, I may be terrible at nonverbal communication. Sure, I may be unable to cry over the death of someone I actually know personally. Sure, I may be terrible at remembering that most people don't respond well to having their mistakes pointed out. However, I don't allow those things to be a disadvantage, they even come with the advantage of being rather good at math and proofreading computer code. I don't let my mental state be a handicap, simply a difference.

Now look at all of that stuff that's been said about Luffy. Does he let those things be a disadvantage? No! Infact, it even seems to help him. His childish nature that even shows up in some of his attacks (like Gomu Gomu no Mikata Robo) seems to leave him more willing to try things that others would simply dismiss as stupid or dumb. His simplistic mind even seems to be more suited to reaching a state of "mushin" (go ahead and wiki it) than those of other people. He's mentally different, but not mentally handicapped.