I'll have to go with Buccaneer on most of what he said.
Granted, having something like someone else finishing the main villain or putting fights "out of order" may be interesting, but I trust Oda to still come out with something exciting and climactic whether he stays the same or goes off on a new route.
If anything, I'd say One Piece often may appear "predictable" in this aspect, but the way it's delivered truly makes up for it.
Now, if we're speaking of anime, I'd say yes, there is a bit more to complain about, such as filler, overly long recap, badly drawn scenes, etc., but these are general animation faults and should fall to Toei, not One Piece in general (as this is in the General section).
And really, stuff like "characters make a sacrifice but don't die" never really bothered me that much. I've said it before: Someone attempts to give up everything to save everyone, and you think their just reward is to have them die?
I go off and read, say, FMA (which is a pretty fine series), where stuff like this actually does tend to happen, and yet, I don't like it. Sure, some death, when used carefully (and sparingly) can be good for a series, but having someone do something great and then having them just be killed-off erks me more than having them survive (and don't forget, this IS a Shounen series =p ).
I think the thing that bugs me the most about OP is the fans. During the saga, inbetween all of the Usopp angst, theories about Spandams shoes eating Devil Fruits, Pauley bickering, and complaints about the same thing every week, this board was brought to an alltime low.
Complete and total agreement.