I too am a big One Piece fan, an at a first look, I thought this was taken straight out from the series. I'll start from the top.
Just a minor thing, the suns position and the shadow from his nose are not very well lined up. I know, this is not something you tried very hard to do. If you had it wouldn't be a much better picture, it might even have become worse, I don't know. I am sure you are aware of this, but I will still note on these things. (They really don't matter that much)
One of the things that makes this clearly not taken out from the series, is the clouds. And water, and grass, and wood and all that. They too, don't really matter. It's not always good to make it look just like from the anime. Nothing too notable, except for the clouds. They so clearly do not match the style of the main aspect of this painting/drawing/whatever; Ussop. None of the other background elements do either, but you have managed to hide them very well. The thing about the clouds is that is is so close to his face, the center and main focus. The water is a bit out of place too, but not as much as the clouds, and the clouds are not that disruptive.
I have said this in a previous critique, but I must say it again; "It is very difficult to draw a hat without making it look too tight or too lose, especially hats like that. This one looks a bit lose but it's something you only notice if you look really close." That was what I said, yours looks a bit tight. It looks flat and gives very little evidence of depth. Here is an example. www.psvitawalls.com/thumbs/one… See how thick it is? That is one way of creating depth. Also, you can clearly see that it is turning around, but in your picture, it is face on. I'll come back to that. So yeah, I think a bit of thickness would help.
Now to the most important part, the nose. Remember how your picture is face on, same height, not in frog-perspective? The nose should not point up like that. And it is rooted a bit high I think. Yes, he has his face lifted a bit, but not that much. It may be harder to draw at the same height just because of the fact that his nose doesn't look long. Still, were not talking about much, just like one and a half cm. If you take my example again, and compare their nose lengths relative to their hats, they are about the same, yours being a bit smaller. However, i think it should be even smaller. I said this was the most important part, but is isn't that bad. Perspective is just so important for the artist's sake to work on.
The earmuffs/thing on ears could have had a bit of shadowing to give depth. Depth is perhaps just as important as perspective, so work on that too. His body; shadowing on the neck, hands, clothing, goggles are all very good, but work on details like earmuffs and hats too.
His hair is fine, it's supposed to be all smooth and unedgy. (best word ever (Don't use it in a exam)) His soul patch(beard) however, is not. It looks a bit strange and is just a detail to keep in mind later. (I don't know all this, i look it up and check as i go, if you want to make a super serious painting, you should too)
I see you have added an extra line on the goggles to make depth and that is good. That was the thing the has was missing. However, here too, you hadn't planed all the way ahead, and so you have to end the lines, and suddenly, they loose their depth.
Just some quick details: A bit too hard lines on the end of his palm, hands are difficult. Maybe he should have a bit more muscles? You need details to make the picture more interesting. The most common one is mussels. The example is again a good example. The big white wall thing is also a bit boring, here your imagination comes into play. I am not good at drawing at all, but i know that the railings to the left don't look like they should. They look as if they were parallel with the floor. You really need more shadowing there. The railings to the right are fine, but I'm not entirely sure what they are standing on. If you gave that a bit more detail, just some lines, it would look better.
I have said a lot, that does not mean there was a lot wrong. It means you are good enough to improve on a lot. Great picture!