What is it supposed to do? I click on it and it gives me no real options, no extra pop-ups, nothing.

The plus sign isn't for additional options, but to give the post a +1. Posts with a certain number of upvotes get the "high rated" mark.
Until quite recently you could also give negative ratings, and the calculate between positive and negative was calculated, and posts would also get "low rated" marks and threads with low rated OPs wouldn't show up as unread, and after low-rating a post you could also report it as spam and threads with many reports could even get automatically deleted, which opened the way for abuse, so that option was eventually eliminated, as was the public display of a user's reputation. That's just cosmetic though, as the reputation system still remains, you can check yours under "My Settings", and experience shows that scripts may still be used to low rate posts.
But with an account created 12 years ago, this shouldn't be news...

Haha, thanks. I've been clicking/experimenting on the "rate this post" thing not understanding what it does (thinking it might have some sort of pop-up) so I guess I've inadvertently marked a bunch of probably-not-that-helpful posts as helpful. Oops. Anyways, even though my account is 12 years old I am not active at all on the forums. Too many trolls. I can see how that could be abused. I actually came across a troll and I was trying to downvote his comment; I guess it's better to just ignore it then. Thanks!

I gave this post a +1. Seemed only fair :D

Yeah, this is not cool. I wanted to down vote a spam/ad message, that had no relation to the thread, and i ended up up voting. I can't take it back either. Really nice, well thought out feature.

I've been a member for a long time as well but I probably don't have any activity in the forum. That being said, it shouldn't matter how new or old you are. The feature is bad and i'm not going to remember the workarounds.