At Pisidian Antioch the typical pattern of the Pauline ministry was established: an initial proclamation in the synagogue to Jews and Gentile adherents and then, when refused an audience in the synagogue, a direct ministry to Gentiles. This pattern is reproduced in every city visited by Paul with a sizable Jewish population — except Athens." (Acts, 422)
I think Longenecker is spot on, for the most part. The extended account in Acts 13 establishes a basic pattern of ministry that is invoked in abbreviated form in every subsequent account of Paul's experiences in the cities of the Mediterranean. Luke, I think, does this deliberately as a way of instructing the reader on appropriate reading strategies (I'm incorporating here some of John Miles Foley's appropriation of Iser's and Jauss's Receptionalist work; cf. Foley 1995:42ff.) by which we can more faithfully read his account of Paul's proclamation. This is why I think Longenecker is spot on only "for the most part": If Luke intends for us to incorporate this pattern into each account of Paul's activities (with, of course, the explicit modifications to this pattern that Luke provides), then even Paul's experiences in Athens may incorporate this pattern (cf. Acts 17.17a). This is a narratological suggestion rather than a historical one.
This point becomes especially important, I think, because some scholars have suggested that Luke espouses a supersessionist theology in which God has abandoned his old people, Israel, and has chosen his new people, the Church. But this fails to notice that, in every possible instance of Paul's ministry (i.e., even in Rome [Acts 28] but not in Lystra [Acts 14]) Luke presents Paul as ministering explicitly to Jews, even after his caustic exclamation, ""It was necessary that the word of God should be spoken first to you. Since you reject it and judge yourselves to be unworthy of eternal life, we are now turning to the Gentiles" (Acts 13.46). In Luke's presentation, Israel continues to be at the heart of both God's people (see, inter alios, Paul and Barnabas) and God's mission (e.g., Paul's proclamation).
As concerns the importance of Galatia as an aspect of "the world of the NT," I note two things. First, as already suggested, the pattern of Paul's ministry throughout the Roman world was established (again, at least narratologically speaking) in Galatia. This might be a minor point, but I want to connect it with another point (perhaps equally minor): Famously, Luke's account of Paul's and Barnabas's experiences in Lystra (i.e., in Galatia) is strikingly parallel to his account of Peter's and John's experiences in the Jerusalem Temple in Acts 3–4. In terms of Acts' story, then, I wonder if these observations can be pressed to pry open Luke's perspective on the interaction of continuity and discontinuity between (a) Christianity as it developed under Paul's influence in the context of the wider Greco-Roman world (again, in Galatia) and (b) Christianity as it originally found expression as a Judean (or, in Luke's presentation, as a Jerusalem) sect.