You can't really leave comments. You can reblog something and add a comment, but this requires you to reblog it, and you might not want to reblog everything you might potentially comment on, so that's a hindrance. You CAN "reply" on some posts (that end in a question mark) if the person who made the posts enabled replies, and even then their settings might stop you from replying if you are not following them or something. But yeah, the main forms of interaction are a) reblogs, b) likes, and c) ask box. None of which are really the same as a comment. (And people can also disable others from leaving them messages in the ask box, which is not linked to specific posts anyway.)
I basically use my tumblr to post whatever random things I am enjoying at the moment. Mostly I use it to reblog pictures I like. So I use it more for my own personal enjoyment rather than for anyone to view/follow it. I guess it is more like a personal diary which I do not intend for others, whereas my DW/LJ are more focused on interacting with friends. (However, that is definitely not how everyone uses their tumblr. Many ppl are focused on sharing things or on getting as many followers as possible.)
There is a large fandom presence on there, but the predominant types of activities are not those on DW/LJ. Mainly there is a lot of photoset/gif posting/reblogging. Many of the ppl who do fandom things on there are new to fandom, may or may not have a DW/LJ account (many don't), and want faster content delivery (new pictures on every refresh).
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I basically use my tumblr to post whatever random things I am enjoying at the moment. Mostly I use it to reblog pictures I like. So I use it more for my own personal enjoyment rather than for anyone to view/follow it. I guess it is more like a personal diary which I do not intend for others, whereas my DW/LJ are more focused on interacting with friends. (However, that is definitely not how everyone uses their tumblr. Many ppl are focused on sharing things or on getting as many followers as possible.)
There is a large fandom presence on there, but the predominant types of activities are not those on DW/LJ. Mainly there is a lot of photoset/gif posting/reblogging. Many of the ppl who do fandom things on there are new to fandom, may or may not have a DW/LJ account (many don't), and want faster content delivery (new pictures on every refresh).
In addition to fandom-oriented tumblogs (of which I don't think you share my fandom interests so you probably wouldn't be interested), here are some trans/queer/kink/porn tumblogs I like:
http://nakedpeople.tumblr.com/
http://leathermen.tumblr.com/
http://thewomaninsideme.tumblr.com/
http://queerlysf.tumblr.com/
http://prettykinkythings.tumblr.com/
http://freshlycharles.tumblr.com/
http://dishevelleddomina.tumblr.com/
http://333images.tumblr.com/
http://colorfulqueerdom.tumblr.com/
http://amosmac.tumblr.com/
http://ireensarrows.tumblr.com/
http://tiemeup.tumblr.com/ (sadly this one hasn't been updated in like a year)