If there WERE a sort-of "anti-DC bias" in the minds of movie reviewers, how do you think it works?

[Asita]

The second is just a sort of shift in the comic book fandom's tastes towards more upbeat and colourful movies, which the DC films decidedly aren't.

I think it's this more than anything else. The bulk of critics, and possibly the general public, seem to want movies that are more in line with Marvel's style of being bright, colorful and on the more lighthearted and comedic side of things.
IT seems to me that any movie that goes for a darker, more serious and less comedic stance gets harsher reviews from critics and the general public.

That's a bit harsh, don't you think? There is considerable difference between being "serious and less comedic" and going for dark and edgy. In the case of DC there is at the very least the perception that they have been leaning more towards the latter than the former.

For the sake of example, perhaps the most memorable response to the early Dawn of Justice trailers that I saw was - and I quote - "so dark that you need night vision goggles to see it!" And it's not terribly hard to see where they were coming from with that. You had Batman who seemed to be reading out of the Justice Lords' playbook and running around with a gun. Then there was an apparent allusion to Jason Todd's death thrown in for good measure, and Batman himself being driven by the effective "Endor holocaust"[footnote]You know, remnants of the second death star raining down on the planet and pretty much destroying the biome[/footnote] of the climactic battle in Man of Steel...to the apparent end of murdering Superman in cold blood. With regards to Superman, there was the whole "false god" thing and tossing around the idea that he'd go rogue (bringing to mind Legacy from the animated series), Doomsday (Death of Superman) and a "pollution inspired" Aquaman. And of course all of this is brought by way of Batman Returns. Are any of those necessarily bad things? Heck no. In isolation any one of them could have worked spectacularly. You could even have made them all work if they'd been spread out over multiple installments. But throwing them all in at once? And making that the focus of the promo material? That's not "more serious, less comedic", that's gleefully skipping past that line and then frolicking into "Look at how edgy I am!" territory.

 

[bastardofmelbourne]

(again something DC aren't getting right, why have a TV show about the Flash, produced by your TV branch and then use a totally different actor for your movie version??)

There's two practical concerns here. The first is scheduling; the actor playing the Flash can't spend half a year filming Justice League while also filming the TV series.

The second is that trying to keep track of a shared universe between television and film shows is actually kind of hard. Even Daredevil, which I'd consider the gold standard for superhero TV, has caused a couple of continuity snarls that Marvel can't be happy about - for example, they used up the Ben Urich character right before Marvel got the rights to Spider-Man back. And according to Joss Whedon, Coulson is dead and Agents of Shield doesn't exist.

Aside from that, there's quality concerns - I have seen literally one good superhero TV show, and you get reading comprehension points for guessing which one it was - and thematic concerns; they might want to do something with Flash on a stylistic basis that contradicts the TV show. For example, Film!Flash's speed effect, according to Justice League footage, is some kind of time dilation that causes an electrical discharge (a literal "flash", nyuk nyuk nyuk). If they used TV!Flash, then they'd have to duplicate that effect in the show itself, which would be hard with a television budget.