For lenses:

• overall build quality and durability (how long the lens will last)
• smoothness of zoom and focus, zoom creep
• materials - metal or plastic barrel and mount, glass or plastic lens elements
• optics
  • number and design of elements (two similar lenses may have a different number and configuration of elements and this may affect the performance of the lens)
  • lens coatings to eliminate flare
  • distortion, chromatic aberration, vignetting (darkening of the corners)
  • contrast and sharpness throughout the range of apertures, both centre and in corners
  • bokeh (subjective quality of blurred areas of the image)
• autofocus speed and accuracy
• compatibility with current and future camera bodies (3rd party lenses are to some degree reverse-engineered, so no guarantee any and all communication between the lens and body will be equivalent with a 3rd party lens
• image stabilisation (VR/OS/IS)
• internal focusing (IF)
• ability to take filters and filter size (larger diameter = larger, more expensive filters)

There are some very good 3rd party lenses (Tokina 17-50mm, Tamron 90mm macro). Overall 3rd party lenses tend to be less expensive but with lower build quality. I don't think you can generalise - there are some very good Sigma lenses, and some poor ones. You'd need to read reviews and try out these lenses yourself to know which ones are good value.