Well, this one ended up very long. But considering all of the things happening this arc, I guess there is no avoiding it
Edit: coming back here a few days later, god how much did I write?! Well, the most important thing is the underlined words - the rest is just me babbling.
~ The Capital of Water. One Piece has an amazing world filled with a variety of intriguing islands, but few beat Water 7 in pure beauty. The fountain was an amazing idea, and I like the subtle reference to Venice.
This was the first actual city in the series that genuinely stood out. Before this arc, the creativity of environment was mostly restricted to nature. Alabasta, Loguetown, Windmill Village… most settlements were very "standard" - the closest we got was Angel Island and Shandora, of which one was only a temporary stop and the other abandoned ruins.
The Capital of Water is a bustling city with an equally bustling economy. We learn how the inhabitants live, what the city is famous for, its history, its culture... I dare say of all towns the Strawhats have visited so far, this is the by far most explored.
~ A darker plot. If there is anything I remember when I think back at this arc, its seeing the Strawhats fall apart. We've seen a fair amount of drama before, but this time I actually went "Oh shit, they aren't actually splitting up, are they?!". I was genuinely worried - and evoking such a feeling in the readers is not an easy thing to do, both in terms of pulling it off and in terms of actually having the guts to do so. Its one of the times when Oda's writing really shines. Whether its a loathe or love, there is no denying the crew splitting up had an impact on most readers.
Water 7 was also responsible for uncovering the true nature of the World Government, which should not be overlooked considering their current status in the series.
~ Franky. Franky is one of my favorite strawhats, and this is the arc that introduced him. I love how you absolutely hated him at first, but as time went on you couldn't help but laugh at his antics. His talk to Usopp is also one of my favorite moments, along with his flashback.
~ Character Developement. It was nice to see some character development, especially since the two with the most in this arc were Usopp and Luffy. All of a sudden you started to take Usopp seriously, and this was also the arc that made him one of my favorite characters. And then there is the fact that Luffy actually had to embrace his role as a captain. Most of the time, this involves "having fun, doing nothing, beating the big bad". This time Luffy proved once and for all that he IS a capable captain and was forced to make a hard decision, and that even he can become distressed and depressed (and its not a gag).
~ A memorable cast of characters. As the recent character evaluations have proved, the side cast of this arc was very memorable. There were no characters that were genuinely boring. There weren't too few, nor were there too many.
I'm a huge fan of exploring side characters and taking a breather from the main crew, and after having to endure the not-so-enjoyable side characters of the Alabasta and Skypiea arcs, this was more than welcome. I loved reading about Galley La, about Iceburg and his secretary, about Kokoro, even about Franky. They helped lighten the mood while the Strawhats were dealing with their issues - and when they were betrayed from within, you felt for them.
Loathe While I don't think of Water 7 as my personal favorite arc (that spot has been stolen by Dressrosa and Drum), I do believe it is by far the most solid. It had just the right balance of characters and story. So I can't actually come up with a good loathe.
Actually, I found a bit.
The train fights.
They had some moments, but mostly felt just dragged and uneccesary. The only memorable villain there was T-Bone, the rest were pretty forgettable (though we did get that one moment when Sanji used knives).