+1 action:
Moat - a very weak $2 card.
Laboratory - a moderate $5 card.
Difference? Laboratory gets +1 action.... and even loses the reaction!
+1 card:
Village and Fishing Village are both $3 cards. (Ok, yes, Fishing Village tends to be stronger).
Difference? Fishing Village gets +1$, as well as +1 Action and +1$ next turn! The only thing it has to sacrifice to get all those extra benefits? +1 card. (Ok, and the fact that Durations miss the reshuffle more).
Also, compare Festival to Grand Market.... Festival has the extra action, and costs less, and doesn't have the buying restriction! That +1 card on Grand Market is huge.
Just $1 more:
Lighthouse: only $2
Merchant Ship: $5
In order to produce $2 this turn and next instead of $1 this turn and next, Merchant Ship loses the +1 action, loses the defense ability, and costs much more!
Anyway, no real point to all this, I just find it kind of fascinating how a seemingly small thing like +1 card or +1 action can so drastically change the power level of a card.