What keeps users here is not features but the quality of the links and the comments. So that is what I spend the most time on. E.g. we have really good software for catching spam, and also semi-spam promoted by voting rings. There are also a number of subtle tweaks to prevent flame wars.
The last code I released (the day before yesterday) was to enable the system to ignore the flags of people who flagged too indiscriminately. Which in turn depended on code I wrote to study how big a problem bad flagging is. Like good defense, this kind of work is generally invisible. Even news.ycombinator.com/classic is something I wrote for myself to try to gauge whether quality was declining.