In most places, the "wall of monitors" use a VGA splitter to turn one signal into many. In most cases, these stores buy the cheapest (read worst) video splitter they can find. If they had 30 monitors to display, they may have to daisy chain a number of smaller video splitters in order to get the picture on all of them. Every time you split the signal, there is some degradation. Add to that the above mentioned cable length/quality, non native resolution, and possible interference; and you get a crappy signal.
What I would recommend is that when you want to buy a new monitor, do some research online. Find out the key characteristics like native resolution, dot pitch, response time, and manufacture warranty. I usually use newegg as a good place for reviews. Just make sure to take everything with a grain of salt.
In reality, the most important thing when buying an LCD is knowing the return policy for the store and dead pixel policy for the manufacture. It's normally hidden and may require an e-mail to the support folks, but this can save a lot of irritation later. Here is what you should expect for a manufacture policy. Note that after the first 30 days, you can get up to two dead pixels in certain areas and you have to live with it. That's actually a lot stricter then some other manufactures (comparison here). When it comes to the vendor return policy be sure that you you buy is what you want. Most will return defective items without incident, buy you may have to demonstrate the problem. So be ready for that.
In my experience, quality LCD screens (high native resolution, low response time) perform much better at home then in the store. As with all consumer electronics, the more you know, the better your purchase will ultimately be. Be wary of "great deals" and "sales", don't buy something because it is cheap. A good LCD can last long past the warranty and the life of the computer it is bought for.
Hope that long, rambling answer helps you out.