I don't remember any mention of slavery on Ferenginar. If you mean that half of their population were second class citizens (women), Bajor had a caste system, so it's exactly like it was an example of equality, either.
Humans in Trek seem to be respectful of other cultures as long as they're the same as their own. A most jarring example of a prejudiced and contemptuous attitude towards another culture was in fact the attitude of Picard and others to the 20th century Humans in "The Neutral Zone", all the more puzzling because they should have shown a bit more understanding for people who were thrown into another time and had experienced a huge cultural shock. What is worse, the show didn't portray it as bad, as they did with Stiles' racism towards Romulans and Vulcans, or the prejudice towards androids that popped up in TNG from time to time, or O'Brien's issues with Cardassians; the 20th century Humans were portrayed as stupid backwards bumpkins who deserved to be despised. A similar, if a little more covert, attitude is shown towards various aliens in seasons 1 and 2 of TNG (see my thread about 'Stupid aliens'): the TNG crew tends to either preach to them ("We were like you once, but we have evolved..." type of speeches), or regard them in a dismissive if slightly amused way, or underestimate them (as with Pakleds in 'The Samaritan Snare'), but those aliens are made to behave in such ridiculous way and their cultures are made to look so absurd, that you can't really criticize the TNG crew for it. But you can criticize the writers for their inability to portray a culture different from a 24th century Federation as anything different than primitive and misguided. (Which would thankfully somewhat change in the later seasons of TNG and on DS9.)
I've also noticed that there were many moments when TNG Humans had a slightly dismissive or mocking attitude to Data, particularly in the early seasons. It wasn't just when the show was intentionally drawing attention to it, as with Pulaski, or the guy in "Redemption". Lots of times you'd see Riker and the others snickering and exchanging grins at something that Data said (which the audience was supposed to participate in, since Data's lack of understanding of human behavior was a source of comedy), or they would be impatient with him and cut him off when he would start reciting the facts. As if they found him endearing, but kind of silly, like an eccentric cousin that you like having around, but laugh at behind his back.
See also the way that the crew treated Barclay - which showed that they're not that different from the 20th century humans as we know them, after all: they will still ridicule anyone who doesn't fit in with the majority.
And I'm not saying that this is a bad thing to portray: it shows that the 24th Humans weren't perfect and that they were still very human, even if they were more PC than the folks of TOS. But it's incongruous to insist that those people are perfect or that 'evolved' in comparison to their ancestors.
I'd also say that comparing them to Trek aliens and going on about how worse the attitudes of those aliens are is, well, not the best way to establish the superiority of Trek Humans. "Yes, they may not be perfect, but those other guys are worse..." That's like those people in real life who constantly defend the actions of their government by pointing out that there are others who are worse. Shouldn't people be trying to meet the standards they set for themselves, rather than justifying themselves by using the worst possible standards for comparison?
And since we're not talking about real life but fiction, these aliens are like that because they were written like that, and they were written like that exactly for the purpose of making the Humans look better by comparison.
Trek occasionally tends to indulge in "Humans are the most special and awesome beings in the galaxy and everyone should wish to become more human" attitudes. One of the most jarring examples is "When the Bough Breaks", when Picard tells the aliens that Humans tend to have strong feelings for their offspring - which implies that Human attachment to their children is something extraordinary among sentient races of Trek. Well, we don't have real life experience with other sapient species, but someone should have shown the writers of that episode some of the nature documentaries, like this one, that show certain animals showing deep attachment to their offspring (like the doe in this video - at 3:10 - devastated by her death of her fawn).* Then there is ENT "Observer Effect", which implies that, out of all the sapient species that the Organians had ever observed, Humans were the only ones to show compassion. Seriously?
*Although that might not have been the intention, one could see Madred's words to his daughter in "Chain of Command pt 2" ("Human mothers and fathers do not love their children as we do") as putting those comments from "When the Bough Breaks" into an ironic perspective.
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