I think this is a big issue that affects both tank balance and enjoyment; how incredibly passive their items are. Looking at things like Thornmail, Abyssal Mask, Frozen Heart, Sunfire-- these items just do not feel *good* to finish because there's such a little noticeable effect and none of them have a condition deeper than "be near the target." or "get hit." There's no interaction here; you simply get more for being in soaking distance of someone else, and there's no condition to it or a cooldown beyond that. And I think that's a big contributor to why the work around to tank items has been just adding so many direct counters to tank items that they feel even worse to buy right now.
It doesn't necessarily need to be out onto actives, but they certainly need to be more engaging. Compare this to other items that go to other classes. Steraks is limited on a cooldown and the main effect it has (its shield) is *very satisfying* while still having counterplay; you do not want it to pop from being poked and whittled down, because then you don't have it when you get into the thick of a fight. BC has to stack up, it feels good to hear that SFX pop and you know you've done something useful. Duskblade has the caveat of attacking from out of vision and it feels good to use. Ludens has to charge up, and it feels good to pop off. Even if these items can be big stat sticks, like Infinity Edge they're inherently more engaging and satisfying as offensive pickups because they require the player to be proactive.
And because defensive items are so passive they can't actually be strong effects. Thornmail does like 3 different things on its passives right now, slowing AS, doing return damage, and applying Grievous, so in return its effects have to be completely unsatisfying or else the item is too strong. Abyssal Mask had its range almost halved to compensate for being 100% uptime debuff aura. Etc.
Put these items to more engaging uses. It gives you more room to make these tank items stronger and more fun. Let your usefulness be more visible and clarified. Actives with cooldowns, conditional passives, stacking passives, any of those are great workarounds to making them better for the game. And then we can dial back anti-tank effects to compensate for windowed tank powerups. Everybody wins.