Potential ways to nerf Shield, and my thoughts:
Make it Concentration - While a very definite nerf and encouraging an important in-game tactical decision, I do not think that this addresses Shield's core issue.
Triggers when attacked rather than when hit. - Some DMs reveal whether Shield would actually stop the hit or not, making Shield much more powerful at those tables. Deciding whether to cast Shield before you know it will actually stop damage for you requires a lot more thought.
Change the calculation. - Rather than +5 AC, Shield grants a flat AC of 13 + Spellcasting modifier + Proficiency bonus, with no other bonuses applying. - This is my favourite I think. It makes Shield still a useful spell, but doesn't allow the stacking with armour, bladesong, or other bonuses that makes it so powerful. I do not have an issue with Shield granting a good AC. I think the problem lies when Shield is combined with an already high AC to give an AC unachievable any other way.