You're absolutely right, and it's one reason I don't really miss Tumblr as a platform. (I left about a year ago and never returned.) Tumblr is great for shiny things — gathering nice gifsets, picspams, or other content about your favourite fandoms — but it's not a great place to be enthusiastic at length about things that really resonate with you. It moves too fast for that kind of verbose enthusiasm to sink in, or for people to engage with it. Dreamwidth is a much better platform for that, and also, as you say, for picking up new fandoms.