There is a cost to Guidance, but it's a marginal one: the Cleric needs to choose that cantrip among the three that she can cast. That's probably not enough.
Good point. I'd say it's an important cost, so let's rephrase my issues with Guidance: it has a
strategic cost, but it doesn't have a
tactical cost.
There are other things in the game that have (or could have had) no tactical cost. I'm not sure right now for example if
Mage Armor can be cast as a ritual: if it can, then it has a strategic cost (learning the spell instead of learning another) but no tactical cost (no daily slot used) or a normally irrelevant tactical cost (the time to cast). I would have few problems with that.
Still, the case of
Guidance just strikes me as wrong... everyone is going to get that +1d4 every time. Out of combat of course, and only when you have time to cast it before the check, but it's still a lot of occurrences. I don't like the idea of playing a game where the Cleric's player continually says "I cast Guidance, I cast Guidance, I cast Guidance..." because it's free, and neither I'd like to make it automatic.
I agree that it should be used more than it is, but some curbs are appropriate. The mechanical check you suggest (no retries) is, I think, an obvious solution. I am also partial to an RP curb, not for the cleric (for whom the religious schtick is to be assumed, but for the recipient of the spell -- make him say "Guide me, Yondalla" or whatever -- people are proud, and it would certainly help curb excesses at my table.
A RP-type of requirement is a very nice idea. I was thinking about making guidance applicable to tasks that are directly linked to the deity's portfolio/dogmas, but I can't figure out how to do that sensibly... Saying "we're on a mission for Tyr" is not specific enough to get Guidance from a Cleric of Tyr, because it would once again apply to every check. But how do you link checks with the ideals of "justice" or "law"? There is the opposite risk that it will rarely apply at all.
(Of course the solution of the problem would be very simple: make
guidance a 1st level spell, eventually increasing the bonus if too low. But that solution is botched already)