Diversity and homogeneity
(I like Joe's quote from Montaigne: "There were never in the world two opinions alike, any more than two hairs or two grains. Their most universal quality is diversity." I'd forgive Joe for quoting a deist humanist from the French renaissance, but then I'd have to forgive Paul for quoting pagan poets on Mars Hill.)
If you're interested in such things, it may provide something of an explanation how church groups have become segregated, not only according to race in some cases, but according to sets of doctrinal opinions (such as written or unwritten creeds).
In ruminating on these things as they relate to Christian unity, it's important to recognize that neither diversity of thought nor uniformity of thought are stated goals of Christian association. Paul did not command us to "speak the same thing" on every opinion--that is humanly impossible--but to be in harmony with each other in our love for the Lord. Nor did he say to try to make our discourse as cacophonous as possible.
The mutual edification of believers, always in humility and love, is the stated goal of associating with one another. It is true, as Joe pointed out in reference to bloggers, that we will tend, over time, to gather into similar groups. For believers, however, there are clear dangers in either extreme as far as group dynamics goes. For the Christian to form a spiritual bond with anyone and everyone who loosely professes Christianity is not good, because there are clear reasons (continued immorality or denying the deity of Christ, for example) that require us to separate from a brother.
On the other hand, we can be so conformist on the other extreme that we never allow fresh (and potentially correct) thoughts about the Scriptures to challenge our previous thinking. What's the saying? "Ego is an anesthetic that dulls the pain of ignorance." I must have needed pretty high doses of that anesthetic at times.