En masse bicycle commuting
It all depends on the riders that you encounter, actually. I've been in group rides of 40+ where everyone was an experienced cyclist, and it was smooth sailing. People were able to move in and out of various parts of the group at will, road safety and laws were observed, etc.

I've also been in rides with 6 people where the handling was sketchy, they spread out across the road, ignored traffic laws and it was generally not a pleasant experience.

The "commuter culture" is pretty much going to be just like the car commuters. You'll have quite a few that obey the laws and operate in a safe manner, then you're going to have the few that ignore the laws, people around them, and generally make it unsafe for everyone else. My favorite lately was a college "hipster fixie" rider that was riding no handed, cigarette in the right hand, cell phone glued to the ear with the left.

Operate your bike in a safe manner, watch out for the cars and the unsafe riders, and you should generally have a pleasant commute (And yes, there is nothing better than zipping past a long line of stopped cars! :D )