The Nikon D40 is a great camera. Firstly, the 18-55mm lens is very sharp, and is an absolutely great lens to have. Great for landscape photography. Just because it's ' variable aperture, plastic, and rickety ' doesn't mean it's not a good lens. It doesn't have the best build quality- but it's a good lens nonetheless.
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The D40 Line of cameras are very simple to operate, and that is what people starting out appreciate. Oh, and it is not hard to change the shutter speed of the aperture. I change both in the Manual ( m ) mode, and i do not find it a problem at all. The A/F is only slow on the 18-55mm lens, but of course you can get another, faster lens. I do not find the camera laggy or unresponsive either. I really do not know where you are getting your information from.
I own a Nikon D40, and a Canon eos 30d, which is a prosumer camera. I really think that in SOME cases, the D40 is better to use. The colors you get are better with the D40, and the light meter is much more accurate. The D40 is a very good camera. I do admit that the 30d is better in a lot of cases, obviously. The D40 is a very good camera, not worthy of you talking down to it.
You have to remember that the camera is an entry-level camera, and is not appealing to a professional buyer. It is marketed towards people just started, and looking to get the best results for their money.