Which resolutions have the big differences that you can tell? I know there is a huge between 480p and 1080i.
Again, the difference here is in the source. A standard DVD has 480 lines of resolution baked into the recorded bits. Therefore, when you play this back on a TV with 1080p resolutions (1920x1080), it has to take the 480 lines and then double them (480x2= 960) and then has to come up with 120 more lines. However, if you watch a broadcast that is recorded at 1080i...it has 1080 lines of recorded information so when you play back at 1080 or 720 on the display...it either stays the same or is reduced slightly. But since you aren't making up something that isn't there, but rather cutting some of the lines out...the picture does not noticeably get worse.
So, in my case.....all things being equal. On a properly calibrated 720p display and a properly calibrated 1080p display...if you were to take a 480P DVD and play it back...it should look the same. It's possible with crappy electronics in the display that the scalers would do a poor job going to 1080p...but if the make and model used the same components with different outputs it would be the same. Same thing if you took a 1080p BluRay and played it on both. They should look the same. Even the one at 720p is going to look great....because the bits are actually in the source and just being reduced at the display. However, if you compare the DVD on 1 to the BR on the other...the BR will always look better because the source has more bits.
With modern TV's, they are fixed pixel displays. Meaning a 1920x1080 TV is always showing 1980 lines by 1080 lines at all times. In fact, they are just tiny dots. Unlike old CRT TV's, they don't show what is being given to them, they turn whatever they get into 1920x1080. So, when you take something that is 480 and bump it to 1080, quality goes down (like copying a cassette tape from a cassette tape), but when you take something that is great at 1080 and play it back at 720p, it still looks great.
Hope that makes sense...I'm trying to keep this at easy to understand levels....but sometimes what I think in my head doesn't translate to words well.