We already knew that Thanos was a big deal. Though we haven't seen any of his actions specifically happening, we know of his reputation and Nebula bears the marks of his sadism - not to mention that tasty looking punch that knocks Iron Man clean out in the trailer. What we didn't know is why he really wants the Infinity Stones, other than a vague hint that he courts their power.
It had been assumed widely that we'd see the same story as the Infinity Saga comic book event, which saw Thanos use the Infinity Gauntlet to impress Death as a means to woo her, but surely that'd be too weird for Marvel?
Now we know a little more of his motivations thanks to EW and Kevin Feige, who revealed the most complete bio we have of the Mad Titan, and it adds a lot of depth to him:
“He’s from a planet called Titan that’s no longer inhabited because of things that he thought he could help prevent, and he was not allowed to do that. What he feared most happened, and the planet and everybody on it basically went extinct. He vowed not to let that happen again. He thinks he sees the universe going down the tubes. He thinks he sees life expanding outward unchecked. That will bring ruin, he believes, to the universe and to that life.”
Having villains with this sort of complexity is a big step forward for Marvel: they've taken the same approach with Zemo and Erik Killmonger and they were too of the most successful in the MCU. And while Hela's motivations weren't as personal thanks to her theatricality (and Ragnarok's tone), the same mentality was certainly there.
So that'll be a good way of adding more complexity to Thanos, and actually a little more empathy. And it definitely sounds like he's going to kill a LOT of people if he gets all of the Stones, since his agenda is one of mass-scale genocide.
And according to Joe Russo, his quest for the Stones will make Infinity War a sort of heist movie:
“He’s on a hunt. We’re using a bit of a ’90s heist genre component. Thanos is on a smash-and-grab, and everybody’s trying to catch up the whole movie.”
The same EW feature showed off an image of Thanos using the two Stones he picks up first to smash an actual moon down onto his enemies. Which cannot be good for their health.