"For the purposes of these rules, an action is defined as any time a player plays a card (including marshalling), or chooses to use an ability on a card already in play, including responses. " (Bold mine)

"Important exception: All card abilities that are preempted by the word Response: are specifically not actions and may not be triggered unless allowed inside an "Action Window" (see below)." (Bold mine)

What am I missing here?

Yeah not sure why it says that first part. Anyway here it is from the core rules "Taking an action is de?ned as playing a card
(including the playing of any card during the
marshalling phase, by the active player, for which
a gold payment is required) or using the ability of
a card already in play (responses do not count as
actions)."

So responses don't count as actions.

The "important exception" statement is in the section describing a "player action." So what it is saying is that a Response is not considered a "player action" in that it cannot, on its own, open a "player action window." Responses are not "player actions" and you do not "take an action" when you trigger them.

You could consider them "actions" in the sense that they are things players choose to do. That's what the first part (which is describing the "many different types of actions" players use during the course of a game) is getting at; Responses happen when players want them to, not when the game says they do.

Yes, it could be worded better, but from the context, it isn't so hard to make some sense out of it.

ktom said:

The first above sentence attempts to state a definition of a term "for the purposes of these rules." If we are required to interpret that very term in a different "sense" elsewhere in the rules, then I think the sentence is, at best, a failure.

I don't think it would be going to far to say the first sentence is not true. (i.e. "for the purposes of these rules" the term "action" will be used in a different sense later on.)