But common carriers do have nondiscrimination obligations, while sites that benefit from §230 don't.

I believe that is only to account for editorial policy in a UGC context.

I'm not sure I understand what you're referring to there.

Edit: I mean that being prohibited from editorial interference seems like a pretty long distance from being essentially completely protected in editorial interference.

To be sure, it's not "completely protected," as we've seen with The Pirate Bay, etc., but Common Carrier says the phone company indeed can't refuse to rent you a phone number just because you want to run a sex line on it. That YouTube has protection against people posting videos of themselves speeding or doing drugs (it's always "vices," natch) was a necessary consideration in Sec230.

I guess there are arguments to be made about how far apart these might be conceptually, but what would the country/world look like without Safe Harbor? We may be about to find out.

I'm a huge supporter of the safe harbor and I hope different kinds of communications intermediaries will continue to be free of legal responsibility to know or control what people communicate through their services and platforms... I was just questioning the conceptual part in your observation that the common carrier regime was the inspiration for §230, which I don't see as straightforward because of this difference about content discrimination.