[Derekb]

Build quality, AF speed and sharpness. Makes no difference whether you're a pro or not, there are differences that can clearly be seen. Whether that matters to you is dependant upon just what you do with your images.

I started out with kit lenses (the Nikon 18-55mm and 55-200mm - IMHO the worst lenses I've ever held) and while I got some good images, it was the images I missed that made me upgrade. Now if my images are not sharp or soft in places, I have only myself to blame.

That doesn't mean that all 'cheaper' lenses are crap though, as there are some real gems out there. The Nikon 18-70mm is a great little lens and the Sigma 10-20mm is the dogs gonads.

Edit. I will also add, one of my sharpest lenses is also my cheapest. Get yourself a 50mm prime for as little as £50 - money very well spent.