First you need to determine if you want a portable or stationary saw.
Stationary Saw:

• how wide is the right and left rip width.
  
• Do you need table extensions.
  Are outfeed extensions included or optional.
  
• How easily can you read the measurement gauge.
  
• How easily can you adjust the saw, height, bevel, fence, miter gauge, out feeds, etc..
  
• Does it provide a dust collection port.
  
• What accessories are available.
  
• Is the power switch in a convenient location; can you get at it fast in case of emergency.
  
• Safety features, does the guard look sturdy? Some saws even have a feature that stops the blade if it comes in contact with a finger.
  Check reviews on saws also.
  

Portable Saw:

• The power is one consideration if you plan on cutting hard woods or pressure treated lumber you want a higher horse power saw.
  
• Tool storage is also important do the tools store out of the way for hauling the tool around, do they lock in solid?
• Does it come with a stand, how easily does the stand fold up?
• Does the stand have wheels?
• Is there a dust bag for the saw?
• Also regard many tips from stationary saws.
  

New Used Or Reconditioned: This is another option, new and reconditioned come with warranties.
used are cheaper but you probably will get no guarantee.
I've bought quite a few new and recon.
tools over the years and have found that they work equally well.
Always remember you get what you pay for.
If you buy the cheapest model of most anything the quality will be equal to or less than the price.