I raced D2 in college and now will be starting my first season on a UCI professional road team racing in North America, Asia, and Europe(raced from category 4 to 1 in a season). Without getting into the difference between mtn bike, cyclocross, and road; I'll throw out why I think there are major differences between road vs. running.
The running community is a lot more low key in the sense that as some previous posters mentioned; talent is very easily visualized. If you're the best you can drop just about anyone around you. Sure they may make it for a bit, but eventually it's going to get to them. The masters runners, lesser athletes, slower runners, whatever you want to call it, know their pace and don't try to venture from it (usually). And if they do try to throw down, you'll be able to hammer them into the ground after a good uptempo (if that's your objective). Cycling is only similar in this sense, when it's a long hard hilly day or you're absolutely murdering the front, you can drop someone, but it's a little bit harder on flats.
This is where the huge difference comes to play. In cycling, everyone thinks that if they make the right move or do the right tactics, they can catch the other off guard, even if they're a category worse. It's a lot easier to "keep up" with someone on a bike. Especially if it's a 30 mile ride. I'll ride to a "fake race" and guys will try and murder each other for the hour. But the difference between me and them is that I ride the 25 miles there and back, while most of them park their cars at the venue. Sure they can make me hurt on the flats, but they can only make those efforts for short periods of time. And thankfully, for some odd reason; being on a pro team gives you some sort of super powers in the eyes of local racers. Don't ask me why, but they are very respectful now.
Of course you'll have your masters who are "elitist" and will tell you how good they "used" to be. But that's because they can actually keep up for some of these rides, so they're around long enough to hear their jabbing. Or you'll have your cat 4 with the $3000 wheels because he has the money to spend. But in my opinion, while a bit comical; it is what it is. The only thing that really makes me angry is when I'll ride to a group ride, and guys either a) ride dangerously or b) don't respect the rules of the road.
Therefore, I mostly ride alone and with friends who I know aren't going to attempt to measure the size of their junk every second of the ride. While more rampant in the cycling community, I remember a ton of group runs where it was the annual smash each others' heads into the wall.
With that said, cycling is a lot more dangerous in a group. If the riders in the group aren't aware of their surroundings or don't care, then I'll get really upset. That is one big difference I've noticed. When I was with runners and you would start to get carried away; it wasn't 30 guys lining up across the road. It was 5-7 guys hammering side by side single file.
Once you get out of the categories and into the professional ranks, I think the camaraderie is very similar to collegiate/professional team running. You have your sprinters, climbers, all rounders, TT guys etc. You work together, you're friends. It's just like running in that it takes a team to win the meet or race. So at a professional level, I would consider the "community" extremely similar.
There are great people in both communities. I think the running community is a little bit less "crazy". But I do find the personalities at the upper level, extremely similar.