Here's a list why the dubbed version is crap.
The theme song was supposed to be "We Are" the opening to the uncut OP translated to English. But it was replaced with a rap theme song which sucks a lot.
All the great orchastral music was replaced with lame pirate music.
For the voices, Luffy sounds like he's 5 years old, Zoro (who's now called Zolo to avoid lawsuits) sounds to old, Nami is okay, Usopp sounds like his balls are being squeezed at all times, and Sanji sounds like a thug with a head cold. All the merman in the Arlong arc have echoing voices. Pearl from the Don Krieg arc is a surfer dude. Shanks is Irish because he has red hair.
Luffy's gun related attacks were changed. Gomu Gomu no Pistol is now Gum Gum Blast.
Many useless edits that even three year olds can see through. An example is the spring-loaded hammer in episode 3 which was supposed to be a gun.
There's many more but my fingers are getting tired from typing.