Opening ten minutes aside, I wasn't bored at all during Avengers. I was bored constantly during dark knight.
Can't really argue the Cameron thing, there are some serious clangers in Avatar.
it's not that the dialogue is bad in dark knight just that it's lifeless. People speak in statements to serve the story. I remember one scene between cat woman and Batman when she gets on the bike: it's about the only scene where there's a bit of humour and banter between them. That's why I preferred*avengers. It's a dumb film for sure, but every scene is full of lovely character moments and it's those that make me want to watch a film again.
it's also a tonal thing. Avengers got away with a certain amount of cheese because you were expecting it and whedon writes it so well. The tone in dark knight is so serious that the superhero stuff just jars and seems stupid.
I'd happily watch any of Nolans film again but the Batman trilogy just baffles me.