The lack of offline support is a deal-breaker for me using Checkvist. I don't have the greatest internet connection and when I've tried to use Checkvist in the past, the page frequently drops the connection when trying to sync and then when it reconnects it dumps what I've just added/changed.

Time is useful for situations far simpler than detailed day planning. For example, I have a whole bunch of tasks that can't be done until the evening (say, after 5pm). If I could make them due at 5pm, then they would show up when I can actually deal with them and not spend the whole day cluttering up my due list.