Problem solving: Poor image quality
This is not an easy topic because many things may cause you to become dissatisfied with the quality of your images when viewed using a viewer. Obviously if you are not happy with your image(s) before putting them on a web page, don't expect them to suddenly look any better!
Image looks pixelated or "blocky"
The screenshot left shows a panorama that looks terribly pixelated and blocky. The reason is that I "accidentally" made the viewer load the thumbnail image rather than the full-size one. Since the thumbnail is only 96 pixels high (whereas I have set this viewer to be 180 pixels high), your web browser has to scale the image up in size, which causes the pixelation.
Always make sure that the image you are displaying is at least as large in pixels as the viewer will display, otherwise you will get pixelation.
Image does not animate smoothly
The smoothness of animation (for both Panoramas and object movies) depends most on the size of the image(s) and the power of the computer being used to display the images. In other words if you try to load a very large panorama on a low performance machine, expect the panorama to not pan smoothly! Some viewers have quality control settings that allow you to adjust how precisely they display the images - the more quality you ask for, the slower and jerkier the image quality on older computers. Note that the speed/efficiency of viewers varies - one viewer may perform noticeably slower than another. (An example of this would be comparing JSphere and PURE Player, both Java viewers, but PURE Player is much faster.)
For panoramas there are different types of projection also demand slightly more or less computing power. The simplest kind of viewer is a projection-less viewer, in which you seem to be looking through a small window onto a flat image. If there are curves in the panoramic picture, there will still be curves when you view it through a projectionless viewer (an example of such as viewer is PMVR).
Most viewers use cylindrical or spherical projection to re-display the panorama as if you were really there: curved lines in the panoramic picture will look straight again and normal when viewed back through the viewer. Good examples of this are ImmerVision's PURE Player (used on this website), PTViewer and QuickTime.
Image does not look as sharp as the original
Unless this is caused by the image being an inappropriate size (see pixelation above), this can be down to your choice of viewer and the settings you use with it. Be sure to check whether the viewer defaults to low or medium quality and whether you can specify a higher quality setting.
Most viewers will reduce the image quality slightly when the panorama is rotating (causing a shimmering effect), and fully-render the image only when it is stationary. The best viewers allow you to specify full quality even when rotating, but this may cause problems on older lower power computers that cannot cope with the calculations involved.