Truth is, no edition yet has really captured the shield properly. There's a reason that "weapon + shield" was one of the most common fighting practices.
And I can see why it's difficult. Shield gives too low an AC bonus, and it's not worth it. Shield gives too high an AC bonus, and it's a no-brainer that overshadows all other options. And the gray area in between is tiny.
On a purely mechanical level, I'd love for shield to give a small AC bonus, but also let you make a roll once per round, in response to a (physical) attack, and if you equal or exceed that attack's roll, you've deflected it.
In practice, of course, this just adds another role and "interrupt" to the game, so I'm not suggesting that this is the way to go. But I do think it models the shield more accurately than what we've seen.