INTRO
When near an interactive world object, "Use default action" is the key used to perform actions associated with that object upon aiming at it.

EXAMPLE
The mechanic goes like this:
You approach an openable door - you aim at it - an "open door" icon appears - you press whatever key assigned to "Use default action" - the door opens

ISSUES
At the moment, there are two issues with the "Use default action" key compromising the interaction mechanic:

1. "Use default action" brings up the Action menu even when there is no interactions available.
2. "Use default action" works the same as "Use selected action" once there is an open menu.

Couple these issues with the fact that the interaction system can be quite erratic and elusive at times (the icon indicating an available interaction appears and disappears depending on how you approach the object or move around it) and the above example ends up going like this:
you approach a door in a heated situation - you aim at it waiting and praying for the "open door" icon to show up - the icon plays hide and seek with you - you press "Use default action" key more than once trying to catch the icon - the Action menu is recalled because you pressed "Use default action" at the wrong time - since you were pressing "Use default action" repeatedly, you end up switching weapons, reloading different ammos, placing mines etc. - you swear

The following are two other examples taken from this thread http://bit.ly/16I1otf on the BI forums:

"[...] when on the move (either you or the world object you interact with - famous hovering chinook entering videos in A2), you get the problem that the action can disappear quicker than you can react, still pressing the space even though now it switches your weapon."

and

"ATM there is no keybind that allows do the "action" without bringing the "action menu". THAT'S THE POINT. Give us a button that does "action" connected to object we are pointing on AND ONLY THAT. I don't want to click RMB for x times to dismiss action menu, because this ladder I wanted to climb up on has an illusive hitpoint."

SUGGESTION
The first step to improve on this would be to make it so "Use default action" works ONLY in conjuction with available interactive actions and never ever bring up the Action menu when there are none at your disposal (the player should see the Action menu pop up only when scrolling the mouse wheel or pressing whatever key assigned to "Next" and "Previous" in Options > Controls > Common).

Also, once there is an open menu, "Use default action" should not behave the same as "Use selected action" since they are two separate controls (they should work the same only if assigned to the same key).

Obviously the final step would be then to make the icon-based interaction system more reliable and less of a pixel hunt, but this is for another ticket :)