+Traits, +Slots, different class definition syntax and its processing, different file API, full block closures, different source file syntax, ByteArray literals, different comments syntax, many differences in the standard and UI library, etc. No, Pharo is not Smalltalk-80.

I think incompatible Smalltalk implementations were one of the bigger barriers to Smalltalk adoption. In contrast, Java standard libraries have been a huge benefit.

One thing that comes to mind is that Smalltalk-80 doesn't do traits, or the UI is completly different.