Another open-source contribution, this time from Jeff Kayser.
There are now two buttons at the top of the comments section, next to the header that says "X comments". The minus button will collapse all the reply comments, leaving only the top-level comments expanded, and the plus button will uncollapse any comments that have been collapsed.
This lets you browse the comments in kind of the "opposite" way of normal—instead of collapsing the conversations you don't want to read, you start out with only the top-level comments visible, and can un-collapse responses if you want to read more.
There will probably be some more work done along these lines in the future. A user setting would probably be nice so that reply comments are always collapsed by default, if that's what you prefer. And I'd also like to see the collapsed comments say how many comments there are in that chain, since right now there's not really any way to tell how large of a discussion you might be un-collapsing.