The contrary of unity is not diversity but division, just as the contrary of diversity is not unity but uniformity. Unity and division, like uniformity and diversity, were reciprocally exclusive, but there was nothing to prevent unity from coexisting with diversity--and in retrospect one might even say (indeed, should say) that, if unity was an imperative grounded in the gospel itself, diversity was a concrete human, existential, personal and cultural condition of this unity.Now, whilst I do take issue with Meyer's failure to observe the Oxford comma, nonetheless this is remarkably brilliant. It is one of the best, most powerful, kinds of brilliance: a set of simple heuristic distinctions that clarify concepts too often conflated. Unity refers to a single set, whereas division refers to a multiplicity of sets; diversity refers to a multiplicity of non-identical members of a set, whereas uniformity refers to a multiplicity of identical members of one or more sets.
Let us think about this in relation to recent and current debates surrounding the ontic status of gospel communities. Theories regarding such communities developed in part to account for the diversity among early Christianity communities. The problem is that, using the heuristics introduced above, this is not really what they do. Rather, they posit the existence of an early Christianity that is divided into more or less doctrinally uniform communities. There is a Matthean community that is divided from a Markan community that is divided from a Lukan, a Johannine, and Thomasine. Even if some diversity is acknowledged within the community it is highly circumscribed: different keys of Matthean Christianity might exist in the same community but surely not Matthean and Johannine Christianity.
The difficulty with this picture is that it does not quite line up with what we see in the data, most notably but not exclusively those found in the Acts of the Apostles and the Pauline letters. The first Christian community, i.e. that gathered in Jerusalem before even Paul's conversion, is presented not as uniform but as diverse, yet despite this diversity united. In fact, this unity is a necessary condition for the diversity, for if diversity is difference among members of a singular set then we must be dealing with a singular set. Likewise Paul's discussion of "Petrine" Christians and "Pauline" Christians, out of which Baur built so much so tenuously, indicates a distinction not between Christian communities but rather within Christian communities; so too his discussions of class distinctions, gender distinctions, etc.
Insofar as we can talk at all about distinctively "Matthean" or "Markan" or "Lukan" or "Johannine" or "Thomasine" communities I am not clear why we should expect that they would be much different. Rather than divided, more or less uniform, communities (which despite this division somehow still circulated among themselves enough of their textual products as to generate the Synoptic Problem) we have a united, diverse, Christianity present in multiple locations throughout the Mediterranean and probably points east. Rather than merely local variants we have a more a complex picture of local instantiations of trans-local tendencies. Now, of course, there might well be regional variation, as Bauer aimed to show in Orthodoxy and Heresy, albeit with mixed results due to a number of conceptual and methodological albatrosses, but that would have to be studied on a case-by-case basis, from what we know about particular regions (cf. Lampe's impressive study of Christianity in Rome, or Tom Robinson's of Christianity in Antioch, or Paul Trebilco's of Christianity in Ephesus).
The interesting thing that comes out of this is that, despite initial rhetoric to the contrary, Bauckham's critique of the gospel community paradigm does not lead to a more homogeneous Christianity but rather a more genuinely diverse one, for instead of islands of homogeneity we would have a sea of heterogeneity. And that seems a much more interesting and in fact realistic picture of earliest Christianity.