Well, I'll try to be as brief as possible in order to not drag this for too long for both of us.
1. Originally, the strowhats (and I mean the original 4 members) were based on classical archetypes.
- Luffy=King archetype
- Zoro=warrior archetype
- Nami=lover archetype
- Ussop=rebel archetype (although Usopp actually combines multiple archetypes)
- Sanji=wizard/magician archetype
Each character had its own perks and features that made them who they were.
- Luffy was slightly naive "some times little bit dumb," courageous, responsible (and yes, Luffy was responsible back then), wise like he was in Alabasta, kindhearted etc.
- Zoro was brave (as a true warrior should be) slightly irresponsible, mildly rude, little bit on the edge, loyal etc.
- Nami was cute, kindhearted, mildly narcissistic, smart and devious etc.
- Ussop was genuinely funny, smart, kindhearted, tricky
- Sanji was chivalrous, smart, very kindhearted, very wise, very responsible, calculateive, loyal etc.
And obviously the gags...
- Luffy=dumb
- Zoro=topographical cretinism
- Nami=narcissistic
- Ussop=slightly cwordness
- Sanji=slightly perverted
But in post-timskip Oda decided to change their characters in order to show the changes during two years timskip. But, there were two problems with that. First, Oda made those changes without actually showing the timskip (outside of small flashbacks) which made it too sudden and unjustified. Second, Oda did it in order to make strowhats more a fanservice motirial then an actuall characters. So, what happened?
Luffy: from slightly naive to retarded, I mean completely brain dead. From responsible to absurdly irresponsible, now Luffy is amongst the most irresponsible characters in one piece alongside characters like Caesar Clown.
Zoro: from slightly irresponsible to very irresponsible (although not as much as Luffy), from mildly rude to insultingly rude, from little bit on the edge to falling from the edge, from loyal to slave.
Nami: from cute to lewd, from mildly narcissistic to extremely narcissistic, from devious to malicious.
Ussop: he didn't change much outside of his design.
Sanji: from chivalrous to servant, from slightly perverted to sexual harassment, from smart to dumb, from kindhearted to angel, from wise to martyr, from responsible to slightly irresponsible.
That's a small sum of what has led to post-timskip outcomes. But the biggest issues that I have with post-timskip are WCI and Wano. These two arcs singlehandedly destroyed one piece with Wano being the last nail on the coffin. Wano and WCI is a completey different topic on its own, and I've covered them extensively in my posts and threads, I might collect them together and create one single thread after Wano.