Whenever I see HD TVs advertised, I will see two TVs with the same screen size, same manufacturer but three or four hundred dollars difference. Is it the screen ratio? If so, why make a TV with different screen ratios? Could the ordinary viewer distinguish between the two ratios just by looking?

Can you give us a real example?

Quite often you will see 2 or more HDTVs of the same size from the same manufacturer that have different feature sets that may alter the cost to the consumer for each. This is typical.

Usually what you're seeing is two different sets of specifications, for example:

TV 1 has a 10,000:1 contrast ratio, 60hz refresh rate
TV 2 has a 15,000:1 contrast ratio, 120hz refresh rate

Or one TV may be 768P native resolution while the other is 1080P. There are a huge variety of items that can contribute to differences in pricing for the same size TV. The differences can be a lot more than a few hundred $ too. When I was recently looking for a 40" Sony, the model prices ranged from just over $1000 to over $2500 - more than double the price for the same size LCD from the same manufacturer. In larger sizes, the differences can be even larger in $ terms. See:

http://www.digitalhome.ca/forum/showthread.php?t=76082

More expensive TVs might also do a better job at displaying SD content, or have more connections available.

Or they might just be more expensive.

All main specs being equal, the difference is in the "feature set". More connectors, menu options, "better" remote control, media player functionality, etc etc etc.

Also, make sure the added features and options are actually stuff that you need/use. No point in paying extra for options that you won't benefit from.