None of those are limiting factors for me. Most new L lenses are 82mm filters, that's not really that big anymore. If it delivers better IQ than the 17-55, which I like a lot, AND lets in more than a stop more light, I'd buy it, even if it is bigger and heavier. Price... I'd pay an extra 20% premium for the IQ and 1.8 aperture.
See for yourself: http://www.the-digital-picture.com …p=736&FLIComp=0&APIComp=0
I can't explain why this is the case, from a physics point of view. It just generally seems to be true...
To go from an 18-35 f/1.8 to an 18-50 will likely cost a lot more than a 20% premium. I'm guessing ( so please bear with that) that the difference in cost might be close to double. And further keep in mind this is a crop lens only! I don't think anyone is going to come out with a $1500 lens that only works on crop. Just my opinion.