The thing is, the Hobbit has been traditionally sold and viewed as a children's book. Therefore, people are more readily willing to think of it as being more light-hearted than it is, or even to search for some element to point at and say "See, look there! It's got stuff aimed at kids!" and say that the dwarves are silly like in Snow White or something. And, though it didn't go down the road that most kids books do (ie insult the intelligence of the reader and pander to the idea that kids couldn't understand complex ideas) and therefore is still appealing to adults, it is a rather light-hearted story. Not to say that it's not serious, simply that it's not Heavy. What they're doing is not World Shattering (or at least, they don't know it is). It's got some pretty scary stuff going on, and there's a huge political environment going on around the events of the story, but it's not about the politics, or the saving the world, or the bad. It's about a guy who's never been outside his own county being swept off on an amazing adventure into a world in which he is very small, in all senses of the phrase.
And, the last time I read it, the dwarves were a comedic element of sorts, from time to time. Slightly black humour, and more situational, absurd humour than Witty Comebacks, and definitely not all the time, but humour nonetheless. You try hiring a wizard to talk your way into the home of a stranger, inviting him on a journey, throwing a party, eating all his food, breaking his things, and then sing a song about how you're doing all these things that this person hates. See if I'm not laughing about all this.
Also, most of our main cast are dwarves. I doubt Jackson would be stupid enough to just blanket dwarves as comic relief, given that then most of the cast would be comic relief, and the film would be terrible (Because like it or not, the Hobbit is not a comedy)...
All that said, I stand with you on the desire for serious dwarves, and do hope that this film pulls through...