Okay, "consensus" isn't and shouldn't be an exact synonym for "unanimity," but the way I use it and dictionaries define it is far closer to "unanimity" than to "some people sometimes have similar opinions on something with some overlap as to who has similar opinions and some overlap as to what the opinions are." The latter seems to be how Robert Christgau and Glenn McDonald and Jack Thompson and probably a myriad others are using it in response to this year's Pazz & Jop poll.
I'm raising this issue not because I think we should always stick with the meanings that were in effect back when there were hula hoops but rather because the word "consensus" in its hula-hoop days (and potentially still) does something good that the new, added usage could well obliterate, which is to describe the process or behavior of an entire group, as a group.
That in the previous Pazz & Jop both Christgau and I and a handful of others put Neil Young's Americana in our respective top tens doesn't mean he and I and they have some sort of consensus on the album. We're not acting as a group and our coming together in this way doesn't meaningfully constitute a group (though maybe the ten of us could get together once a year for a party or something).
I use "consensus" in two basic ways:
(1) Regarding how a group makes a decision, to decide by "consensus" means that everyone or near everyone in the room signs off on the decision. Not everyone necessarily will be 100 percent happy with all aspects of the decision: it might be arrived at through discussion, argument, negotiation, and compromise. But everyone is on board with it. If someone disagrees strongly with a position or course of action, that person in effect has a veto. The word "consensus" here specifically and precisely distinguishes this mode of decision-making from other forms of decision-making, such as a vote in which the majority or plurality of voters carry the day; or a decision by a manager, or owner. In a consensus decision, the process by which the decision is reached may include straw polls, but a minority or faction can't be overridden in the way that it can be in a decision by majority or plurality vote or in a command decision.
Decisions by juries are often by consensus. Decisions by legislatures rarely are.
P&J isn't an election or a decision (though it has the feel of an odd combination of election and opinion poll), but you can see how talking about consensus or lack of consensus among the voters does violence to this meaning of "consensus."
(2) Regarding people's opinions or attributes, a consensus would mean something like "the general opinion of a community or group." So if 97% of climate scientists think global warming is real and man-made, then there's consensus. 80% wouldn't be enough to claim consensus (IMO), even if those 80% are right and the other 20% have no good reason to disagree.
That 65% of P&J voters didn't put Yeezus in their top ten (and presumably it wasn't number one for most who did, so let's say that somewhere between 80% and 95% of voters didn't make it their number one (I don't want to spend the time getting an exact number)), shows how ridiculous it is to say that the strong showing of Yeezus is a sign of some sort of consensus. (And it'd just be babble to turn this around and say that there's a consensus that e.g. most albums outside the top ten aren't the album of the year.)
I think the reason that "consensus" has wandered to include a new meaning — vaguely, to note that there are some criss-crossing similarities among some individuals, some things in common — is that there isn't some other shorthand that's available to wave at such similarities. So the word "consensus" gets to be the shorthand, even if this new meaning takes out the far more useful old meanings. But a shorthand is no good if there isn't real, actual consensus as to what the shorthand is short for. If there isn't general understanding, you shouldn't use the shorthand, unless there's at least some common sense of how to take the disagreements further. (E.g., there's certainly no general agreement as to whether Macklemore & Ryan Lewis are real hip-hop, but people know that there's no general agreement here, and using the word "hip-hop" doesn't paper over such disagreements.) In any event — this is a somewhat different complaint — "consensus" is becoming a buzzword, people waving at ideas they've not actually worked out, trying to quickly communicate thoughts they don't yet have.