I guess the age of "slowed down classic rock covers" in trailers is behind us, ditto the "creepy rendition of nursery rhyme", ditto "one single piano note sustained", and now we're back to original versions (chopped up to suit the scene). But if you recall Iron Man had that in the trailers too (most notably Black Sabbath in the IM2 trailer). I'll come back to that later.
As for why Led Zeppelin, various connections between them and classic D&D and the influence of classic fantasy on their songs aside, quoth Stephen Lynch:
I got a big broadsword made out of cardboard
and that stereo's a-pumpin Zeppelin.
It's that time of the night,
we turn on the black light,
let the Dungeons and the Dragons begin.
And that's the connection for me
You say optimistic... I would want something good/that I will want to watch again in 5-10 years (possibly more but let's not get too silly) but this looks like "wow Guardians of the Galaxy made all the money, let's do fantasy version of that" and that is walking very closely to the already dangerously fine line of unintentional parody. The team of thieves line at the start says someone in there got it (those not aware your group being all thieves is kind of similar to everybody rocking up as a sniper in FPS games). CGI looked a bit... unfinished I shall say but also a respectable amount of practical stuff.
Guardians of the Galaxy did not invent tongue-in-cheek, comedy adventure, or any of the tropes it rides; they just happen to be successful so I guess the comparisons are unavoidable. On the other hand the Guardians of the Galaxy movie (the first one), in tone and in plot,
has been compared to a D&D game on the silly end of the campaign spectrum at the other end of which stands whatever resulted in the Malazan book series, from the variety in characters and backgrounds and classes, to the bouncing from dungeon to dungeon, to the absurd resolution of the final battle, so there's some connections there.
On the other other hand, the first Guardians movie was a straight up comedy, and while the D&D trailer has some funny elements, it's very far from Guardians in tone.
We have Chris Pine as the cocky overconfident bard but unlike Starlord, he didn't make INT his dump stat. Clever quips and banter aren't out of place for a class that can
literally inflict physical damage with their jokes. There's an in-universe explanation of why he might crack a joke on occasion, particularly during battles. I don't like the "okay, here's the sitch" narration but it was obviously a frankenbite and not an actual line from the movie.
If we're looking for parallels, we've already had a movie about a quippy, charismatic, overconfident hero cleaning up the mess he caused through reckless pursuit of money and glory: Iron Man.
tl;dr: there is enough room for the movie to have funny moments and elements while still taking itself seriously.
