Blurry renderings could be caused by anti-aliasing not being used properly. I'm not sure how much you know about the process of anti-aliasing, but the way it works is that once an image is rendered, the anti-aliasor (or whatever that part of the render engine is called) scans the file and determines where anti-aliasing needs to be applied. Most of the stuff it finds is at the edges of various objects, beams, etc. Once anti-aliasing is complete, it proceeds to 'blur' those edges, so that the pixelation is not so apparent.
Depending on the level of antialiasing you choose, and the resolution you are rendering at, this can work for you or against you. For example, if you render at a fairly small resolution and you set your anti-aliasing to High, a lot of that 'blurring' will occur, therefore making most of the image seem blurry. Naturally, the higher your resolution, the more anti-aliasing you should use. My personal recommendations when it comes to anti-aliasing in WYG would be the following:
Your aperture size will also affect the blurriness of your rendering. As soon as you set it to anything above 0, the edges of your image will become blurry. The farther you move from 0, the more the blurriness will move towards the center of the image. This has to do with the WYG camera simulating what a real-life camera does with f-stops and all that.