A core and important difference is that Combine is based and built around the concepts of back-pressure (aka. flow-control) as established by https://www.reactive-streams.org (as Tony mentioned in Will SwiftNIO adapt to the new Combine framework? - #11 by Tony_Parker). It makes it somewhat of a "different beast" from an execution models perspective -- even if APIs seem quite similar.
I for one, am tremendously excited for the acknowledgement and solution (in Combine) of the need of asynchronous back-pressure in streaming -- it shows up in many many places after all 