I started reading One Piece weekly during Thriller Bark. By far the biggest difference is the sense of scale. There were less characters, less chapters per arc, we didn't know as much, and I think that ties a lot into the enjoyment. Pre-timeskip spent a lot of time focusing on the Strawhats, whereas post-timeskip has spent I'd say the majority of the time on side characters, and this is all fine and good, but there's A LOT of side characters Oda introduces in these post timeskip arcs. When you need to write half of the Strawhats out of an arc because of how bloated it is, that's a hint you need to cut down on the significance of certain side characters. There were also less chapters per arc, which I think really helped with the flow of pre-timeskip. The Strawhats were always on the move, discovering new places. That's been driven to the side as the stakes have risen, and the antagonists have grown more powerful. I personally don't mind the super long arcs as long as the story is interesting, but I know some get "arc fatigue" very easily.
We also didn't know as much pre-timeskip everything was still a mystery. I remember several "How can you possibly defeat Logias?" threads in the AP forums back in the day. As more questions are answered the mystery aspect of One Piece that drew some fans in loses it's appeal. There's also the fact that some of these reveals have been disappointing for some fans.
Overall though, there isn't a huge difference between post and pre-timeskip One Piece. Pre-timeskip had it's share of pacing issues, the characters are still the same characters. The humor hasn't really changed all that much. It still feels like One Piece to me.
We also didn't know as much pre-timeskip everything was still a mystery. I remember several "How can you possibly defeat Logias?" threads in the AP forums back in the day. As more questions are answered the mystery aspect of One Piece that drew some fans in loses it's appeal. There's also the fact that some of these reveals have been disappointing for some fans.
Overall though, there isn't a huge difference between post and pre-timeskip One Piece. Pre-timeskip had it's share of pacing issues, the characters are still the same characters. The humor hasn't really changed all that much. It still feels like One Piece to me.