The Christian message is "treat others as you would like to be treated": the idea of people having different wants or needs is out of question. Your values are everybody's values, and if people don't follow them, then they are actively rejecting your values, instead of simply following their own. Your religion is the solid centre of the universe, and everybody moves in relation to it: towards, against, or around it. You want to be a good Christian, so everybody must want to be a good Christian. I mean, who wouldn't want that, right? Wrong.
The humanist message, on the other hand, is: treat others as they want to be treated. Take an interest in people. Allow for differences. Be kind, but don't be overbearing. Treat them with respect. Don't ever think you know what people want or need. Or, more succinctly, "don't be a douche".
I'm sorry, but you're making a distinction without a difference in the way 99.9% of people understand and mean that quote. "Treat others as you would like to be treated" can be just as succinctly summed up as "don't be a douche" (or in the immortal words of the Prophets Bill and Ted - "Be awesome to each other").
As you say, you can't know from the outset what people want or need - but you can know that you wouldn't want them to be a dick to you, so you show them kindness and respect and allow the differences you would hope they allow for you. It ties right in with "love your neighbor as yourself." (The real problem is that both assume people care for themselves - it doesn't have much directly to offer the disconsolate, the depressed, and the self-loathing.)
some of that essential content from the bible:
Is this really gonna turn into some sort of proof-texting thing? You realize that the abolition movement in the US had massive religious support, just as the pro-slavery position did?