One example is the apparatus used for focusing. Metal parts are more durable and operate more smoothly than plastic parts. This is a good indicator of quality. If possible, buy the scope from a camera store or other establishment that specializes in scientific instruments. They generally know their products and stand behind them.
Reflectors cost less per inch of aperture - the diameter of the primary mirror or lens.
The greater the aperture, the more light the telescope will gather and the more you'll be able to see in the sky.