I think we Christians, so far separated from our Jewish roots, have the mistaken impression that immersion is a "new" Christian thing.
I can't speak about attitudes toward immersion among the Gentile believers, but it had long been a common practice amongst Jews. Mikveh, or Baptism, was practiced by certain sects on a daily basis, and was often used as a cleansing ritual before or after major events (like for a High Priest before conducting a Temple service, or for anyone after handling dead bodies, or after having sex). Herod's Temple had several baptistries (ever wonder where the Christians got baptized on the birthday of the church? in the Temple baptistries (mikvaot), most likely). (You can google for more info: a pretty good quick summary that I recommend you spend ten minutes reading can be found at http://www.essene.com/B'nai-Amen/MysticalImmersion.htm)
Of course, we don't have such practices in our culture, so immersion presents a much larger barrier to modern Westerners than it did to the Jews of the first century. Thus it has largely been forgotten in our modern world. I'm reminded of Naaman (2 Kings 5) who at first refused baptism, because it was "too simple". (Thankfully, his servant convinced him that he was being stupid, and Naaman did as he was told and was cleansed of his leprosy because of his obedience (not because his skin got wet).) Modern believers have gotten it in their heads that baptism is an "extra" that is not part of the core of the salvation process, and like Naaman at first, have rejected the examples and commands found over and over in the New Testament. The Apostles never told anyone to "pray the sinner's prayer" and to be baptized after they find a good church to join.
To convert to Judaism in the first century, a Gentile would have to undergo both immersion and circumcision. About fourteen or so years after the church was born, when Gentiles started becoming converts, the Jews expected the Gentiles to convert into Christian-flavored Judaism to be just like themselves (until that time, I don't think any Christian even began to think that the church was anything other than Judaism perfected, including the continued observance of, but not justification by, the Law of Moses), and they expected the Gentiles to be immersed and to be circumcised. This of course led to the conference we read about in Acts 15.