Probably the only thing I regularly find myself nostalgic about regarding my college years was regularly seeing my friends and aquaintances on a casual basis, with no pre-plannIng. At the time this was true both of friends in college and friends at home -at home my friends tended to gather at the local Denny's in the evenings, so when I was home from school I could just go see them. I didn't need their phone numbers-heck, I didn't know half of their last names. They were just there. And many friends also hung out at one or two houses in addition to That.
In college, it was the dining halls, computer labs, and sometimes an off-campus house (owned by the college, allocated to a group of students). My friends tended to gather in the back corner of a particular dining hall for meals, so if I felt like being social I could almost always count on having company. We were also computer geeks, so we would run into each other in the dorm computer lounges and the mathlan computer room.
There is no adult equivalent for this. I'm finding with my toddler that the playgrounds and various library storytimes are kind of this way, for stay at home parents, and I've seen people who open their houses on a regular basis for friday night gaming
Or a Sunday night group dinner, but there's not a shared space that is always open (or at least open every day, and late at night), where we might run into each other for a while and just hang out. Nothing like the front porch culture you sometimes read about in small towns or the cafe culture that was idolized in Friends.
I suspect for some people they get that feeling now through online spaces, like WoW, but I'm not inclined toward that. I get a little bit of it through twitter, I guess. But I'd also like to have it for in-person contact.