General information about telescope   

Seeing conditions 
"Seeing" is the term astronomers use to describe the sky's atmospheric conditions. The atmosphere is in continual motion with changing temperatures, air currents, weather fronts and dust particles. These factors cause the star images to twinkle. If the stars are twinkling considerably we have "poor" seeing conditions and when the star images are steady we have "good" seeing conditions. Poor seeing is most noticeable when observing planets and the moon, whereas deep sky objects such as nebulae and galaxies are less affected by poor seeing conditions. On deep sky objects, the most important factor is the transparency of the atmosphere (a measure of how dark the sky is on a given night-determined by clouds, dust, haze and light pollution). Seeing conditions and transparency will vary widely from site to site, from season to season and from night to night.
Some manufacturers of small aperture telescopes would like you to believe that they can routinely outperform larger aperture telescopes because of atmospheric turbulence (poor seeing conditions). Occasionally this may be true on planets and the moon (you can stop down the larger aperture simply with cut-out masks to alleviate this problem), but it is never true on deep sky objects (nebulae, galaxies and star clusters) where maximum aperture is needed.

Portability 
This is a very important factor in choosing a telescope. If you live in a city polluted with lights you may want to transport your telescope to a dark sky location. If you live in a dark sky location you may have to take the equipment out and set it up. Consider the overall weight and bulk that you will be working with. If you are fortunate enough to have a telescope permanently mounted (or set up), then you should consider the largest aperture telescope you can afford (albeit still considering which type of telescope design fits your needs).

Versatility 
Look for a telescope that can grow along with you as your experience and interest expand. Make sure the manufacturer has a complete line of accessories so that your telescope and your fun are not limited by lack of equipment. Most manufacturers offer accessories that may be added on at a later time.
If you want maximum versatility, consider that some telescopes are multipurpose for the following- (1) terrestrial viewing, (2) terrestrial photography with the attachment of a 35mm SLR camera, (3) astronomical observing and (4) astronomical photography (astrophotography)

Quality  
Most manufacturers are reputable and make good quality products. However, even with the same optical design and same type of mount there are distinct differences between similar units. You need to inspect the units and rely on the advice of telescope dealers, educators, members of astronomy clubs or professional astronomers.
Another very important point is the after-purchase service. Does the manufacturer have a technically competent staff to answer your questions? Can you later purchase an assortment of accessories to fulfill your expanding interest? If you have equipment problems, can you get them repaired promptly?
Also consider the type and length of the product warranty.