alt Hacker NewsFeature 2 is really useful. When I land on a technical article I know chances are there will be a nice HN discussion, that may sometimes even refute the main point of the article.
Feature 1 is IMO less useful. It just suggests that people are bad at window management. We didn't like overlapping windows and finding them, so we created tabs inside windows. Then we wanted to see two things at the same time, so we created split views inside tabs (Chrome and Firefox both had this). With a good window manager neither would be necessary. Sadly the community prefers to solve this at the application level rather than the desktop level; I'm currently using Forge (https://github.com/forge-ext/forge) but sadly it's becoming unmaintained.
Not sure if I agree about using windows instead of tabs. I have hundreds of tabs open, and dozens loaded, at any moment. I'm sure that it's possible to manage all the context with separate windows via a window manager, but it would have to be a REALLY good window manager to be able to provide equivalent features of tree style tabs and separate panes and whatever else that something like Sidebery provides, and it is always easier for each application developer to implement tabs suitable for that application's requirements.