The ability to block specific URLs with the web blocker, with the ability to use wildcards or RegEx to match URLs. Chronoid's time tracking is clearly among the best, but its web blocking functionality does lag behind competitors. I believe the Chrome extension LeechBlockNG has the best implementation of this functionality, allowing easy blocking by adding a domain to a list, but also allowing more advanced URL matching with: or * as wildcard, + to prefix exceptions, > to prefix referrers, and ~ to prefix keywords. Other distraction-blocking apps tend to have website blocking that is rudimentary and casts a wide net with simple domain-matching, therefore limiting their usefulness.