The vertical indicator line to distinguish threads is very nice and super helpful.
I personally don't think looking aesthetically "modern" is (or should particularly be) a selling point for something. I hate most modern things and actually strongly prefer the traditional (non-rounded icons, overkill border-radius on everything, a hundred pixels of extra "breathing space" padding within a single viewport, so forth).
Nevertheless, this is very nice and I appreciate it. It's super sleek.