softness: me, glass, or camera?
Well from what I can see... Firstly, with high contrast scenes like this you would do much better shooting RAW. Or at least turn up your DR. As has been already said too, they actually do not look bad for sharpness. As such... its the lens ! If you want a zoom with good range, try the new nikon 18-140. This would be much better, although you can not expect miracles from any superzoom. Thats the price you pay for all that convenience