What always confuses me about something as simple as a "like" button is that I don't know on which "scale" I'm supposed to rate the post. Do I "like" the way it is presented? Was the content useful to me? do I agree with what was written?

So for example when I'm reading a thread in the more technical forums I "like" posts that I learn something from. Generally posts that explain something in a understandable way ("was this helpful?"-button) or even ones that just point me into the right direction. Posts I'd vote down would be the ones that I know are incorrect, in which case I'd also try to answer and address the issue.
But then when I open my own thread with a question I suddenly feel like I should thank everyone who tried to help me, even if he misread my question. So now I'm treating the button like a "thank you" button. In the lounge I'd want to "like" stuff that has an entertainment value to me, like a post in a funny picture thread or an interesting point in a discussion (even if I don't fully agree). Someone else will probably rate the same post in the discussion based on whether he agrees or not.

So by using something as generic as a "like" button you just end up rating arbitrary (and maybe orthogonal) properties on an arbitrary scale and then somehow represent them in a single number.

For the technical forums I'd just go with a "this was helpful"-button and a "disagree/incorrect"-button for which you somehow have to give a reason (basically a variant of the quote button). If you really want it to be fancy you could weight the "disagree" post based on how many people disagree with the disagree . In the more off-topic forums I'd go with no rating at all since you'd have to either make a very complicated tagging system or end up with another meaningless number since there is no way to enforce or even suggest a unique scale to rate on.