I'm pretty sure "I" means interlaced though It's the type of image output. Progressive is like a buffering where it scans the signal multiple times before showing it to you Interlaced is a direct feed hence the lower image quality.

Yeah unless you set the 2 different types of television next to each other you would never know the difference as a common consumer

LCD cannot create a true black while Plasma can. DLP can create all the image spectrum and look great doing it, only problem is the viewing angles are slightly restricted and it's bulkier.