I've had my HD tv for two weeks now and also still have my old regular rptv to compare it to. Both are 4:3 ratio and both are 55". I chose the 4:3 ratio just because our viewing is 95% 4:3 material. Side by side, you definitely see the lines of resolution in the non-progressive tv whereas pictures on the HD are very distinct. I have Charter digital cable and that too is much better. DVDs are the same difference.
So in layman's terms, progressive tv is twice as clear as regular tvs. I'm sure someone will chirp in and give you a better definition. I had been hesitant about upgrading but now will never go back and wish I had upgraded sooner!
Progressive scan is usually associated more with HD ready sets or HDTV sets more than just RPTV's...
Regular NTSC or analogs TV's only display 480i or 480 lines interlaced...Every 1/30th of a second an analog set switches between the even and odd lines of the 480 lines...There for you notice the scan lines....With progressive scan or 480p all 480 lines are displayed at the same time giving you a much smoother, clearer picture....Just about all progressive scan tvs produced today are also HD ready....HD ready can receive HD programming with a set top box...Most HD programming is 1080i but 720p is also used by ABC for their HD broadcasts...If the HD ready set you purchase is only capable of receiving 480p and 1080i, all the set top boxes avialable will convert a 720p signal to 1080i so your hd ready tv can accept an HD 720p sigal..
Also all set top boxes have the ability to upconvert standard 480i ntsc signals to 1080i which while not being close in qaulity to actual HD is a much better picture than viewing 480i on a standard analong TV..
here's a pretty lengthy article about progressive from hometheaterhifi.com. check out the grey boxed area...it has an animation sequence that kind of explains it too. Link Removed