This was originally published as a comment on a open source browser userstyle posted to Hacker News to increase UI density on a few popular websites.
At a previous company, we had strong feedback from sales that prospects thought our legacy, cash-cow product looked old and outdated. They were right, and those first impressions are hard to shake. We lost deals (in part) because of design.
We did a minor design update in a beta release that introduced ~30% more vertical padding in lists. Beta testers (long-time customers) hated it. They were pros. They could not only handle the density, but the density enabled key workflows that the additional padding broke.
We ended up making the whitespace preference-controlled. Defaulted to on for new users; defaulted to off for existing users. Solved the sales problem while keeping our long-time customers happy.
For infrequently-used tools, especially transactional ones (e.g., Google Maps), additional whitespace seems to improve legibility, usability, and ultimately engagement.
For power-user tools like GitHub in which users spend hours of their time every week, I want to see an industry shift "back" to denser UIs. Power users can handle them and typically benefit from them greatly.