Originally posted by Cez Pentax 24-70mm costs around EUR 1300
Canon 24-105 costs around EUR 700
Nearly half the price.
Sure, if you don't consider that the Canon is a ten year old design, and that tens of thousands of copies have been sold, and that the price you referenced is grey market/refurb. Also, it's a stop slower, and really isn't an impressive lens, considering how large and heavy it is. Try searching for the Canon 24-70mm f/4 L IS, it'll return you to your senses.
The elephant in the room here is SIZE. The new 28-105mm looks very compact, also probably quite light, and I'd guess it'll have performance similar to the 20-40mm Ltd: unimpressive wide open, and very good a stop down the road. If the lens really shapes out to be small and light, then I'll salute Pentax for really sticking to their philosophy of compactness.
PS: primes don't count. You can argue for primes all you want, but many people just need a versatile zoom with WR to travel. Changing lenses is simply absurd in some environments, and it isn't like WR primes are abundant in the line-up, either.