Making Trek just dark would obviously be heresy,
Nah, TOS was pretty dark at times when you remember that in the show a planet with billions of people could be snuffed out at any moment from either doomsday weapons, or space monsters, or bored godlike beings.
Not to mention all the energy being or other aliens who use people as meat puppets or just plain toys for their own twisted amusement.
Then there are the things that literally suck the life out of people.
Also the fact that the universe will explode if two guys locked in mortal combat ever both show up in the universe.
Not to mention the the Federation was always pretty close to war with the Klingons which is usually describe as something that would be devastating.
Oh and if your in Starfleet and assigned to a ship that is not the Enterprise chances are you will die a horrible death in the empty void of space. And even if you are on the Enterprise chances are the first time you visit an alien planet you will die and in some cases quiet horribly.
Then there's the fact that everyone is one trip to the galactic barrier away from dealing with a rampaging mad god.
Seriously all the show needs at this point to go totally Lovecraft is the appearance of godlike aliens who either drive you mad or mutate you. Which were probably hiding on some rock out there somewhere.
So what was that about Star Trek being a Utopia again?
This is all true, but, what most people mean by "Dark and Gritty" these days, is depressing and doom and gloom. NuBSG was great, but, it slid into making viewers want to slit their wrists as times. And...that's fine for one choice, but, some folks want to turn every SciFi (And specifically Space opera) into that. Babylon 5 and DS9 and Farscape, all got just as dark at times, but, there wasn't such a heavy depressing thread running through it. Nothing wrong with Dark elements and realism, and nothing wrong with some of the choices being NuBSG depressing, but, every
show shouldn't be emotionally heavy