Personally, my idea of reviving someone, aside from simply using an item, would be for someone to go on a long arduous journey or fight an extremely hard battle. The other way I can see reviving as challenging is if one were to cast a spell on the fallen character only to basically take their place by having little to energy to fight anyone off, therefore having the newly revived character fight off any threat to their weakened savior. 

As for the strength/weaknesses idea, I agree with your view on it. There are a few loopholes to that system, such as wards, enchanted armor, or being able to devour your specific resistant element, like Natsu with fire, but overall, I feel that it works pretty good and can make a difference in battles.