Although I am obviously thankful for anyone who identifies one of my observations, sometimes i would like to thank someone who has gone a little above-and-beyond - either by identifying something particularly tricky, or mass IDing my observations, or IDing something particularly old. At the same time, it seems a little out of place to simply leave a thanks on the observation itself.
Perhaps there could be a button similar to ‘agree’ on all proposed ID’s, and all clicking it does is give the person who made the ID a notification saying the user thanks them for that ID. It obviously might get a little annoying if people were to thank for each and every ID, but i would plan to only use it in special cases.
An alternative might be a button and stat tracked on the users profile, so for example, if someone has just ID’d 15 butterfly species for me, I could go to their user page and thank them there. This way it is less tied to any specific ID, but rather to the user. The profile page could keep track of how many times someone has been thanked. Again - it could get annoying, depending on how often people use it.