I've often heard people pick at One Piece because "Luffy never trains". He (suposedly) gets stronger between each arc without actually training. But I personally don't think he needs to. For a start, as Robin pointed out, anyone with the Will of D is natrually strong, so why does Luffy need to lift weights to develop this already useful gift? Is it even possible? Perhaps he is already at his limit and unable to get any stronger. Perhaps all training would do is mearly tire him out.
Second, Luffy mainly grows by using his powers in unique ways. While it may be filler, Luffy was inspired to make Gomu Gomu no Pinwheel because he saw the Pinwheel on Genzo's hat. He didn't spend hours training to come up with it, he mearly looked at an everyday object and realise he could mimic it in battle. So again, why does he need to train when he also seems to be able to natrually think up abilities either before or mid battle? And can't we classify Gear 2 and 3 as being mearly powerful attacks? Based on the idea that he can create attack ideas whenever he wants, why does it matter that he didn't train to create them?
Finally, I supose people think that each villain is stronger than the last like in other Shonen series. However can you really see someone like Foxy being stronger than Crocodile and Enel? Personally I don't view any of the opponents prior to Aokiji as being stronger. They just have their own personal ability ( i.e. Buggy's Bara Bara,Kuro and Kreig's weapons, Crocodile and Enel's Logia ability) that gives hem a benefit in battle. However, if taken away I feel they'd be pretty much equal. Kuro relies on his speed and claws to finish off an opponent. However Buggy is immune to blades, and personally I don't think he'd be able to keep up with Kuro's speed. Do you think Crocodile an Enel would be a major threat if they didn't have their sand/electric powers? All Luffy does to beat them is to find and exploit their weakness - remove Kuro and Kreig's weapons, find the elemental weakness for Crocodile and Enel's Logia power, prevent Buggy from seperating his body.
Of course this is just my opinion, but the fact Luffy doesn't train for say fifty hours to gain the power to beat his opponent, instead relying on his natrual strength, his ability to create an ability at the drop of a hat and exploit an opponent's weakness and the fact the next opponent isn't stronger than the last is one of the things I like about One Piece i comparison to other Shonen series.
