Edit: On a related note, I find following discussions on Tumblr to be a huge pain, and hope either this improves in the future or that more discussions happen elsewhere.

Tumblr is downright unusable if you want to read a discussion. Unless you are a direct participant, they are nearly impossible to follow, and for direct participants it is only possible because they know what they are replying to. It is obviously intended for those who like to share stuff and not those who like to read. Improving it would require creating a completely different service based on completely different ideas about how to structure discussion.