Why do pictures I take with my Nikon D3000 come out blurry?

I don't have vision problems and I don't suck as a photographer, but every photo I take without Autofocus comes out blurry. I don't like using auto but if I don't my pictures are always slightly off. They look great small, but then I blow them up and it looks slightly blurry, making them unable to be enlarged without looking bad.

I tested it and when I use auto focus, my subject looks slightly blurry, but the photo comes out clear. What could be causing this and what steps could I go through to fix it? Is it worth repair or should I buy a new camera/lens and how can I troubleshoot on which to purchase?

What exact autofocus mode do you use? Single point AF, or do you let the camera choose the AF points?

Could you post a sample shot? Preferably take a shot with a tripod of a page in a book taken from a 45° angle in which you focus on a particular line, preferably the last line of a paragraph. This way a mistake in the focusing mechanism of the camera/lens combination can be identified.

Like i did here:

I focused on the word "had", and as you see the focusing is right on target.

First, have you checked to make sure the diopter adjustment on the viewfinder is set to 0? Its there for people who do have vision problems so if its accidentally been set to -1 or +1 it will definitely affect your ability to manually focus. It will also cause your subject to look slightly blurry even when using auto focus.

Unfortunately, entry-level DSLR cameras like your D3000 are designed for those who only use auto focus. By eliminating the split-image rangefinder from the focusing screen camera manufacturers save a small amount of money which allows them to be more price competitive.
http://en.wikipedia.org/...ing_screen