This is actually what I do for a living, as a BDM in the industry. Much as with speakers, if they sound okay they are good in most comsumer's eyes. That doesn't make them good speakers or even a good value for money though.
There is actually a MASSIVE difference in sets these days, they are all just marketed as the same to try and group the weak with the strong. So people are left buying without any real knowledge of what to look for, just as with PC's and other electronics.
SHARP for instance, is THE leading brand (manufactured by LOEWE in Germany) of their own and many other brands (such as Pioneer) sold under other names. However, the SHARP brand itself is generally a better value for money than products rebranded for others becaus ethey own a huge chunk of Loewe corporation too.
Lighting is actually a MAJOR factor when buying a screen these days, if not the most important factor (well, besides properly calibrating it when installed)
There are many considerations to be looked at before choosing a set. The lighting level for instance, if in a room with bright light, you will find a matte screen is best and reduces glare, however the screen is more susceptible to damage and scratches.
A high gloss screen, usually a glass panel, is much more durable but needs to be in a room with no light reflections or with very few windows.
Contrast ratios also vary a GREAT deal with LCD's, which is why you should [b]never rely on the picture you see in store[/b] but buy based on specifications instead.
First of all, the store will calibrate screens it wants to sell (usually due to strong margins) but they will leave others in 'out of the box' mode instead, resulting in a poor picture when compared side by side.
However, the cheaper, uncalibrated set may actually be far better in many ways, they just don't make as much margin for selling them so they try not to sell them.
"(Samsung and Sony are best)":
I beg to differ, these aren't CRT TV's, in which case Sony and Hitatchi lead the pack, in the LCD world, SHARP is by far rated the best.
Top picks, from installers and homeowners:
1) SHARP (makers of many other brands too)
2) SONY (though pricey as always)
3) SAMSUNG
4) PANASONIC
5) HITACHI
Now onto other KEY factors:
>Contrast ratio: Here's a great guide that explains how the numbers are misleading and often fudged, often also by purposely miscalibrating them in the store, Best Buy is famous for such 'scams' with their flat panels. In short, plasma GENERALLY has better contrast ratio and colour depth and refresh rate, until you get into the high end system.
http://www.practical-home-theater-guide.com/contrast-ratio.html
>Back or edge lighting: With LED TV's ecomign more and more popular, as they actually do stand up against the plasmas quite well. Back lighting is a nice even light panel but costs a lot of money. Edgel ighting is cheaper and mroe popular nowdays but is not very consitent across the entire panel.
This is a great starter for display comparisons: http://www.practical-home-theater-guide.com/LCD-Tv.html
For a greater definition of LCD vs Plasma, look here: http://www.practical-home-theater-guide.com/plasma-vs-lcd-TV.html
EDUCATE YOURSELF BEFORE YOU BUY!!! That way even when not prepared to spend big bucks for a top end set, you still get the est value for money available. I have a friends that ignired all of my recommendations and decided he'd go out and buy whetever he wanted withotu researching specs. He ended up buy ing THRE sets now in 2 1/2 years, one died completely, one had a dead backlight and was too expensive to fix and the other ghosts an image (burn-in that they say doesn't exist anymore, pure BS) from other channels where it was left on overnight.
Don't buy based on what a salesman tells you in store, don't uy based on specs alone, but learn what differentiates one from another.
simply put though, if you buy SHARP, you can't go wrong with most of their own branded sets.
For example: Best Buy up here has a SAMSUNG 40" 180P LED HDTV on sale for $ 1999.99
However they also have a SHARP AQUOS 40" 1080P HDTV for only $1699.99.
Now logically, you would think that the Samsung is the better of the two.
In reality the Samsung simply has added, possibly useless features such as Headphone output, built-in subwoofer (?impossible to get sub frequncies from a 4" driver anyway?)while the SHARP has a FAR better video processor. Quick display change options for auto picture adjustment to DVD, PC, Movie mode etc. That way you can add that movie effect when needed without recalibrating your set, a 4ms refresh rate and a matte black frame.
So for watching movies, the SHARP set is better, for less money. not too many people rely on a TV for surround sound and built-in speaker quality, but that's what sells the Samsung.
Sorry for rambling, it's my life. Just make sure you do your homework and DON'T let anyone tell you they are all the same and buying on price is your best idea. Those are the words of someone who just didn't do any research.
Lastly, pay the extra $100.00 to have a tech come in and properly calirate it, or read up on LCD calibration yourself and take time to set it up properly, they do NOT come out of the box optimized.