It is possible that all the guardian stones will be skill enhancing, that is each has a different set of skills that it 'improves' or increases skill gain in. Since there are ones for 'Mage' 'Theif' and 'Warrior' there may be some for ex. 'Paladin' which has 1h/block/restoration as its tagged skills. I am hoping this is not the case but it is a possibility.
One question I found myself asking was, "Are we going to find all the guardian stones in sets of 3?". Will each instance of guardian stones be set on a 'node' like the ones in the demo? It would be good to give people choices whenever they found the stones, but on the other hand having some stones be all by themselves would increase the travel time between finding them all and make it a bit harder process to find just the right one.
Another thing I have speculated on was the usefulness of these stones. Will their effects ever grant major or minor powers? Will there be Guardian Stone specific perks that act like birthsigns (Steed for example could be 5% faster movement). If they are, in fact, like birthsigns I would expect there to be around 21-24 including the 3 we know about. If they are just skill enhancers then I believe there to be far less(10 max), as there would be overlap.
The ones we know about, while mundane, could be extremely useful. After one has leveled up a considerable amount in their 'class' skills and wants to eek out extra levels from their 'minor' skills taking one of these would ease the pain of leveling a low skill at a high level. Taking a 'Mage' stone as mid-level warrior would decrease the time that you felt gimped with destruction compared to your sword/shield.