Tripwire hooks have several errors in the model files:
On grounded, attached, unpowered tripwire hooks, the metal ring is too low. It's essentially in the powered position.
On suspended, attached, unpowered tripwire hooks, the entire hook is in the powered position. The blockstate file appears to be correct, but tripwire_hook_attached_suspended.json and tripwire_hook_attached_suspended_powered are identical.
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Confirmed

Confirmed for 1.8. It no longer moves with the rest of the hook when triggered.

Still in 1.8.1-pre3.

Still in 1.8.3
This issue is caused by the model files being made improperly, the ring is down even when the hook is not activated, when it is activated a ground-hook moves into where the ring is.
Also (as seen in the attached suspended_vs_nonsuspended.png) suspended tripwire hooks have a slightly different issue, they are always visibly activated, and do nothing when activated.
Tripwire itself also doesn't have model files (a powered block state exists, but not in the blockstate file, won't work if added) for powered versions. This means that the previous behaviour of the tripwire being pulled down (as if stepped on, completing the effect that you're activating the hook) no longer functions, and cannot be added in without the proper backend things being put in.
This info should be added to the description as it is all the same issue, as they were present since tripwire-related models were added to the model system.
EDIT: @Kumasasa, why didn't you add in this info? Suspended tripwire hooks in a permanent 'on' state is certainly directly related, and tripwires not having powered models is indirectly related as both models were improperly ported to the model system at the same time.

I just loaded up 1.7 and 1.6 to compare, and it doesn't appear to me that tripwires ever changed visually between powered and unpowered states. They don't move, they don't get pulled down. Grounded tripwires always lie flat on the ground, the hooks don't pull it up into the air when it's connected. So I'm not sure what previous behavior you're talking about.

Hmm, must've been my mind playing trick on me because it's what I expect to happen.
I still do think it's an oversight, though. It even goes past visually pleasing/artistic freedom because it's a visibility issue for redstoners. If the tripwire block could have separate models for its already existing "powered" state, the proper string movement could be added, or even a texture change (like a laser tripwire that brightened when activated, especially) would allow redstoners to see when a tripwire is activated. Similar issue is that locked hoppers have no visual indication they are locked.
That said, this is a different issue. Would you please mark this issue as 'relates to' MC-60242 ?
Also, thanks for updating the description.

It's what I expected too, which is why I had to load up several older versions to check for sure. Maybe it did at some point earlier in development, or maybe it just seems like it obviously should. Tripwires are very hard to see already, though that is kind of the point.

Well, hard to avoid (or easy to walk into accidentally) is the point.... but seeing activation of tripwires shouldn't be difficult to see. You hear the "click" that lets you know something has been tripped, but actually seeing the activation (like how pressure plates depress) adds to the realization that a trap has gone off (either by you or someone near you).
That, and anything redstone goes beyond trap uses. I think when it comes to any sort of tripwire-based contraption, a faint red texture that lit up to bright red (like a laser) would be an excellent debugging tool to visualize which tripwires are depressed, even incredibly long tripwires.

can confirm in 15w45a

Confirmed fixed for 15w49a.
However something to note is that existing suspended tripwire hooks will still be broken, must be taken down and then replaced in order for the activation animation to appear properly. Please keep that in mind.