The impeding the progress part. I guess to me, just standing near someone's path to the base (or in this case beyond that path) could impede their progress to that base. And I'm considered by most to be an OVERuser of OBS.

Another consideration to throw in here is the age and skill level of the players involved.
For example
Last weekend, doing some high level men's ball, that's not obstruction - that's just a dumb move by the runner and his team will take care of his education.
Tomorrow night, when I'm filling in for someone in a 14&U B level game - the intimidated runner may well be judged to have been obstructed.