HDTV's key feature is denser line spacing and higher overall resolution. This enables you to sit closer to the screen without seeing scan-line structure, filling a wider viewing angle with images. Greater realism results. Haven't seen any studies, but I'm convinced just having a larger surface area of color, say a golf green or baseball diamond, is a physiological visual clue that enhances simulated realism.
Non-HDTV viewing? Depends. IMO, you need a set with a good (excellent is better) deinterlacing circuit (480i to 480p). Cable signal quality varies tremendously around the world. Some non H/DTV channels from NYC's Time Warner Cable, I find, can be viewed comfortably at an 8-foot distance from my 64-inch RPTV (ideal for HDTV). Image quality often depends on signal routing. CNN, for example, has been bounced over numerous satellite links and figures here often have 'fuzzy' edges from the processing. But studio shots from a local 24/7 cable news source are crisp all the time. Also, all the numerous premium movie channels are usually fantastic in resolution (a theoretical 480X720 pixels, matching DVD quality at times). Upclose viewing of non-HDTV, IMO, is helped by the excellent--Canadian-made :)--Genesis deinterlacing microchip in my Philips RPTV. Also, this year-2000 model (64PH9905) doesn't upconvert 480i to 540p or higher to save manufacturing costs, boosting both scan lines and horizontal resolution, as many newer sets do. Such upconversion requires exceptionally good source signals or artifacts will be unacceptably magnified. So, IMO, with good deinterlacing and other image-generation circuits, plus clean signal sources, bigger images are definitely better. -- John