Ok, if everyone recalls we saw some time ago zancrow, someone who claimed to use god-slaying magic in what seemed to be the same context as natsu used dragon slayer magic. What does using dragon slaying magic entail? Dragon slayers mimic the power of dragons in order to defeat dragons (interestingly, the magic itself is not a creation to defeat dragons in itself but rather it bestows the user with powers of dragons). The users body becomes like that of a dragon and it mimics its magic. Now, if we apply the same logic to the god slayer we get that the god slayer immitates the powers of gods in order to defeat gods.
Now, as far as what we saw there are three things which lead me to believe we should take the whole god slayer thing somewhat seriously.
The first is that when zancrow fought natsu we saw that it was plainly obvious that zancrow's fire was superior to that of natsu. This is evidenced by the fact that natsu could not eat zancrow's flames while zancrow was able to more than easily eat the dragon flames.
The second is that natsu was able to use this flame to combine it with his own to make an even stronger flame which zancrow himself could not it. If the flame was not something special it would not have resulted in something which could defeat zancrow that fast (it would have been interesting to see zancrow using natsu's flame to make dragon god's flame however it would seem at the time he either was to arrogant to do that or perhaps since he was still using his magic (when natsu used zancrow's flame he kinda emtied himself) he was not able to combine it).
The third is what hades said about the origin of the magic of the seven kin. He mentioned specifically that the lost magic of the seven kin was close to the source of magic. In this regard we get that god slaying magic, the power of gods, is close to the source. This would be quite significant overall.
Now, even though the word "god" is usually a bit too much I think we have to put it into context. By a god the manga does not necessarily point towards the traditional definition of the word as an omnipotent all-knowing being. The fact that there is a god slayer implies that there is a creature which is being imitated by the god slayer (as is the case with dragon slayers). The name "god" could simply be what enourmously powerful ancient creatures were called in the past. Dragons at some point were said to rule the earth, perhaps before them were this very power beings.
Anyways, I think the whole god slayer thing could easily have some deep implications in the future, specially if zancrow was indeed imitating the power of a creature.

