Morning, everyone. Lots of questions here, so we'll try to cover as much as we can.
The difference between watch and save is that when you watch a thread, the thread is added to your ‘I’m watching’ list and you can then receive notifications via email or web when there are updated on the thread you’re watching’. When you save an OP post or reply, you’re saving just that specific post to your ‘Saved’ list. You might want to do that if there’s something interesting in the post and you’d like to be able to come back and find it again easily in future.
Regarding the positioning of the watch button, nothing has changed there, so it should be in the chevron menu on mobile and at the very top of the thread above ‘Follow topic’ and at the bottom of the thread under the last post on desktop.
For those of you on desktop, we wondered if you are viewing the site in a smaller window? It's possible that in moving from a full window to a smaller window, it swaps you to a kind of ‘mobile’ type view where everything is condensed. When that happens, the watch option goes to the chevron menu at the top of thread like it does on mobile, so it's worth looking for it there, or opening the site up in a large window.
Regarding the watch thread tick box, that’s checked by default unless you change it in your talk settings. If you have that selected when you post on a thread, the thread will be added to your I’m watching list automatically. So that’s a separate mechanism to the ‘Watch thread’ buttons/options elsewhere. Follow topic, by the way, is also different, and allows you to follow all the threads in one topic, rather than just one thread.
Hope that answers everything. Do let us know if anyone is still having problems. We know it's a pain, but screenshots do really help us to work out what's going on so if you're still having issues and can post a screenshot that would be brilliant.