Check the size of the actual pixels. Some cheaper brands, even if advertised as "Full HD" would have bigger pixels than known brands (Sony, Samsung, etc...). Lesser the pixel density, no matter what size, will be cheaper, and video quality not as good as higher priced models.
Also check for glare, bigger screens especially with shiny panel will produce more glare than matted finished panel.
Also check the back lighting used. Newer models use LED, older and cheaper models use CCFL. Brand new TVs with CCFL may look good when brand new but will deteriorate over time.