In most cases, it's easy. The plus/like/star/thumbs-up is basically a "thanks, this gave me a frisson of pleasure". But there remain a lot of cases where it's not that simple. Are you praising the idea? The expression? The courage to make it? If someone writes about how their grandmother just died, does plussing it mean a pat on the back in sympathy, or an admiration for how it was put into words? If someone passes on a funny political cartoon with an amusing (or not-amusing) comment, what is it saying if you "like" it?
As the "rule" suggested below says, there's also a substantive difference between liking something and judging that that it's good, as well as between disliking something and judging that it's not good. Especially when "like" and "enjoy" and "appreciate" and "makes me happy" and "glad I read/saw this" all mean different things, in the positive or in the negative. There are movies I admire without enjoying them (or, rather, I perhaps enjoy their aesthetic composition while finding their message irritating or simply not being happy after I watch them — and the last two criticisms are not, themselves, the same thing.
All of which says to me that we should be commenting about stuff, rather than just plussing/liking/favoriting it. Except the latter is so easy, and actually satisfices in most cases.
Reshared post from +Doyce Testerman
"Most websites treat "I like it" and "This is good" as the same thing…
"… leading to most people on the Internet refusing to distinguish between "I don't like it" and "It's not good".
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