The main difference is what you can say. Neopets is very strict on language, outside events and outside sites. You can't mention Reddit, Discord, Facebook etc on Neopets without fear of a ban. You can mention approved sites like TDN and Jellyneo.
Language is obvious - no swearing on neopets, but Reddit doesn't care. Note to the wise - don't swear here either. It's not as strict as Neopets on outside events/sites but swearing is off limits.
Neopets also bans all mentions of things like mental health, pregnancy etc. Entertainingly, if you get pulled up on it you;ll get a neomail telling you to talk to a trusted adult. Never mind the fact that most of us are now meant to be that trusted adult!