It just occurred to me how civil the language is on Tumblr, in total absence of the FCC or any other regulating body. I rarely use vulgar language on here (relative to how often the words pop into my head!).
For me, it’s because I take pride in my tumblelog as a high-quality representation of myself and my interests. I don’t want to deface something I take pride in. I try to avoid swearing in Marco.org articles for the same reason (with one big exception — note the date).
Maybe it’s related to the broken-window theory, where being in nice surroundings encourages you to keep them nice, while you don’t give a second thought to making a bad environment worse (littering in a dump, peeing on the side of the football house during a keg party in college).
That, by the way, is roughly how I got into photography. Being extremely impressed with OS X when I switched in 2004, I saw my relatively crappy photos on the slideshow screensaver and thought, “I should really take some nicer photos to put there.”
IRC, online game chat, and most forums are such dumps that it’s easy to dump more swearing, hate, and personal attacks into them. But Tumblr is nice and doesn’t seem appropriate to vandalize.