The art has been improving since the Water 7 arc.

I think one of the aspects that can keep this series going is how Oda only gives a sense of "power level" once in a blue moon. For the most part, the smaller battles depend on abilities and creativity, which is why Oda can keep escalation of power under control. While at the same time, he can give a sense of importance through bounties, even though they don't necessarily represent strength. This is opposed to everony in Bleach having power blasts, people with giant chakra levels in Naruto, and epic power levels in Dragonball.