Keeping in mind I have seen VERY few episodes of DS9 and know TNG like the back of my hand (except for whatever episode Im' watching right at that moment, apparently, then suddenly I'm a total noob >.>)
...The way I always viewed it is this: Remember what Kirk said in "A Taste of Armageddon"?
"It's instinctive. But the instinct can be fought. We're human beings with the blood of a million savage years on our hands, but we can stop it. We can admit that we're killers, but we're not going to kill today. That's all it takes. Knowing that we won't kill today."
TNG is all about fighting those instincts in one way. DS9 fights them in another.
TNG isn't naive for having a more isolated episode structure and a group of characters who are hopping from issue to issue on a weekly basis without examining the long term consequences too much (although I feel the need to note: it's not entirely devoid of THAT either. There ARE issues in TNG that rise up to bite them all in the butt later on: *gestures wildly at Commander Sela*).
And likewise, DS9 isn't overly DARK just because it gets to do what TNG didn't: aka actually show those consequences. Sometimes those consequences were bad, and sometimes the consequences were good, but they were always consequences we didn't always get to see.
TNG boils issues down to what is happening right NOW. It is pure ethics and philosophy confronted. Because that's the reality we often have before us and we do not always have time to consider the future. With that in mind, we can only do what we feel is right in the moment. And that's true both in universe, and when you look at it from a meta perspective.
It presents the crew with a choice they HAVE to make while knowing they probably won't be around to see how it turns out. When there are no hypotheticals about what could be or should be, when you HAVE to make a decision, without knowing everything that might happen in the future as a result of it.
“I know, Professor, “What if one of those lives I save down there is a child who grows up to be the next Adolf Hitler or Khan Singh?” First year philosophy students have been asked that question ever since the earliest wormholes were discovered. But this is not a class in temporal logic… It’s not hypothetical, it’s real. Can’t you see that? A man’s life, his future, hinges on each of a thousand choices. Living is making choices.”
It boils down to that one behaviour: instinct. Doing what you believe is RIGHT and how that matters, whatever the consequences. it's both a valid and true philosophy and also a perfectly good reason for Sisko to start TNG frickin' hating Picard, because even while we know what happened wasn't Picard's fault, we know he was making choices in the best way he and his crew could in an impossible situation... even knowing all that, Sisko's wife is still dead. The people we admire and trust and rooted for indirectly led to the rage and pain of a man we're now expected to follow for several more seasons.
I can see why some people found that a tough pill to swallow, tbh.
We're not supposed to view TNG and DS9 as conflicting pieces of media. They're pieces of media that would not exist without each other.
TNG is the Star Trek series that answers TOS's command: it looks up and decides it will not kill today.
DS9 is the consequence of that choice tomorrow, whatever the outcome may be...
And then, the funny thing is, they still have the make the same choice. And so often... they make the exact same one as TNG did before.
