It depends on your set. What size is it?

If you are using LCD, DLP or plasma, the pixels are a fixed size. If the native resolution of your set is 1080 and you were showing 720 then that's why it looked like crap (the i or p doesn't matter when it comes to the shape of the pixels).

Think of your display like a screen on your screen door. A 720 screen has holes in the screen that are one shape and size (grid pattern) and 1080 has different shaped holes, and a different grid pattern. If you lay one on the top of another, the holes don't all line up. That causes aritfacts and fuzziness.

It's why when you have an LCD monitor on your computer, you are advised to only run it at it's native resolution - otherwise it tends to look like crap - fuzzy and with artifacts.

This is why Standard Definition (480i) also looks like total crap on the above HD sets - the holes in the 480 "screen" don't line up with the holes in either the 720 or 1080 "screen". Ironically, the oldest technology, CRT, doesn't suffer from that issue because CRT's don't have fixed pixel sizes.

The problem of fixed sized pixels is why Microsoft invented TrueType (to line up text along pixel boundaries) and why fonts in Safari for Windows look softer (Apple ignores pixel boundaries to preserve the accuracy of how the font should look).

There is one upcoming technology that doesn't have this problem with fixed pixel size, [url= http://www.google.com/search?hl=en&q=%22laser+tv%22&btnG=Google+Search]Laser TV[/url], looks interesting. However, SED looked promising but got canned when they couldn't get the manufacturing costs to be competitive with LCD or Plasma. It wasn't the first time the superior technology didn't win - and probably won't be the last :p