The headline of this section is a joke I've made, how getting the groups together (especially evangelicals and Catholics) feels a lot more like inter-faith than intra-faith (Christianity). I'm active and consider myself on good terms with fundamentalists, evangelicals, Catholics, and at least a couple mainliners and Orthodox, but there's a great disparity between the wings. Occasionally, people wonder why I associate with one set or another. It starts with background; I am more inclined than most to deal with Protestants. My dad's parents are Presbyterians, I went to Vacation Bible School, and a Methodist pastor and family friend, not a Catholic, gave the blessing at my Eagle Scout ceremony. Also, my views on salvation, for Jews and others, are towards the generous side, but I still mandate belief in God and actions expressing that belief (no Unitarian am I).
College students and twentysomethings have a tendency to act very enthusiastically about things, especially new and exciting ones. It's good in some ways, because society as a whole needs energy, enthusiasm, and frantic action occasionally. On the other hand, exhuberance and inexperience lead to excess, dangerous excess at times. Look at the levels of binge drinking, reckless driving, and bullemia among folks my age. This recklessness extends into friendship and worship as well; college Catholic and Christian groups are not immune. I'll describe the issues on both sides.
On the evangelical side, many groups are designed to provide many activities for new members to be around other members; between Bible study, intensive study, retreats, fellowship services, bowling, Taboo, and such, one could easily spend 20 hours weekly. Spending this much time can make the group insular, blind to believers like me who have other faith sources and aren't interested in six activities per week. Furthermore, the seeming necessity of simplifying Christianity to attract folks reduces doctrine complexity to basic tracts like "The 4 Spiritual Laws" or "Have you accepted Christ as your personal savior?" Under these basic rules, the word "Christian" means "born-again evangelical like me, what I know." This leads to casual disregard of more complex theologies, and the famous statement "Catholics aren't Christians!", which any college Catholic has heard or knows several people who have.
On the Catholic side, especially among leadership, defensive attitudes arise. Unlike evangelical groups, the majority of student attendees at Catholic Mass will not participate in other activities; they're comfortable (or lackadasical) with their faith. The relatively few that do join prayer groups or organize food pantries can feel like a remnant. Additionally, Catholic groups suffer the tensions of the universal church, between emphasis on abortion marches and other life issues, soup kitchens and other social action issues, and prayer groups and other individual issues. Evangelical groups, free of denominational affiliation, are often spared these concerns. Small size and internal tension, when combined with the perceived and actual slights from evangelical groups, often make Catholic student leaders extremely gunshy. They aren't interested in conversation and joint action. This wariness extends to those involved in such groups, like I am.
These problems are extremely unfortunate, because the two groups could really support each other on campuses. Both share enough commonality to act jointly in many things; Bible study, service activities, advocacy against non-Christian policies. There are enough enemies for evangelical and nonevangelical Christians, particularly on campus, that neglecting a possible source of aid is wasteful and best and severely damaging at worst.
If you're interested in a more personal discussion of the matter, I talk about my undergraduate experiences in my review of Regeneration Quarterly 7.2, and you can click to go there.