There could be many reasons for this :
You can take most "sales" peoples views with a huge pinch of salt, especially large retail stores. Especially if they don't have the model in stock or don't stock them in the 1st place. Or have old/inferior stock they are trying to shift.
Some people prefer lcd's to plasma, lcd's tend to be sharper than plasma. Plasma tend to have more natural colours and are better with fast motion. There are good and bad tv's with both technologies though. That's not set in stone though, todays lcd and plasma's are alot closer in performance than they used to be.
Also has the tv been properly calibrated or are they using the "factory" settings. The factory settings are usually miles off and a bit of tweaking can work wonders for the picture quality.
How are the tv's connected, do they have 30 tv's all running off the same feed. Are they using decent cables ?
Are you viewing the tv's at the optimum viewing distance?
Store lighting can have a huge effect on picture quality, ideally you want to test your shortlist in home lighting conditions or even a dedicated testing room. Any retailer worth their salt will be more than willing to do this.
There's probably a few more reasons I have missed as well.
The best advice I can give you is to make a shortlist of tv's you are interested in, then get a proper demo in home lighting conditions with properly calibrated tv settings and decent cables. If you have any other equipment already ie dvd player or blu ray player etc that will be used with your new tv then take them along with you and see what looks best on your shortlist. Make sure to view them at your intended home viewing distance. Look at both SD and HD content and take along any films you are familliar with. Test out how well the tv's can handle fast motion as well. And how good/bad the tv's sound is etc.
There are alot of very happy Pany owners on these forums, and I am one of them. I have the 42"PZ85 Pany plasma and sd content from sky looks pretty good on most channels. Upscaled dvd's through my PS3 look amazing and Blu Ray's take your breath away, absolutely stunning.
Only you can decide what's best for you by viewing the tv's your self. Hope this helps and good luck.