I guess I want it dark, but not super grimdark with everything being constant misery and hopelessness. I want the game to deal with tough subjects, eg murder, rape, slavery, drug use, in a mature way, but I don't want the game to just be that. I want it to also show scenes of hope, compasion, friendship, love, all those sorts of things, but without breaking the dark theme of the game. So stuff like, for example, you have the option to spare a man's life and then later the man sacrifices himself to save the life of a child, or you have a plan to defeat the big bad, it's a good plan, it doesn't quite work, it looks like you're screwed, you share a last kiss with you LI or brofist with your best friend companion and then something happens that lets you still make it out of there alive and live to fight another day.
Having humour is also a good thing, if that humour fits with the world. That is to say, rather than having a ton of references to RL stuff or having a quest minigame where you run around kicking chickens or something, you instead have a companion who makes sarcastic remarks about your situation.
For me, ending the game on a positive note it also rather important. I've always said, give me as much death, destruction, horror and mind bends as you want during the story, so long as it all ends on a relatively positive note. I've said before that my preference for RPG endings is the ME2 way of doing things. A culmination of choices where you can either end the game poorly or positively based on your actions, but BioWare has indicated they favour the big final decision way of doing things where you can't "win" the game. They've also apparently become a fan of bittersweet. Both of those are fine, I can live with them, so long as the final decisions are all very different from each other and do somehow take into account your actions leading up to that point, and so long as the sweet outweighs the bitter in the bittersweet ending. ME3 failed to do that, even with the expanded ending the sweet to bitter ratio is still not right, and I hope DA3 is different.