There's lots of LGBTQ+ and FOSS (I wonder why lol). Star Trek and science are also becoming rather common (or it just took me a while to find the right communities). I also get the feeling that the age bracket is larger than on other platforms and people seem to be nicer too (at least compared to the month or so I spent looking through Reddit before finding Lemmy).

Following is more about the kinds of post than users. (I don't want to waste your time if that's not what you meant by "common things people see") There's also some videogame and pornography communities (I'm sure there's combinations too) so I do set my client to blur nsfw images and I block video game communities for games I don't play/don't want spoilers from.

Lots of webcomics also seem to be (automatically?) uploaded to their respective communities.