No, 1080p is not a "gimmick" by definition. However, retailers do try and oversell it by convincing average consumers that they need it... when they don't. Not by a long shot. The thing people never consider when deciding on the resolution of screen they need, is the distance they sit from said screen! The distance you sit is JUST as important as the resolution of the screen.
Most people don't sit close enough to their TV to see any benefit from 1080p over 720p. In general, unless you're building a home theatre, or sit abnormally close to your TV during normal use... you will NEVER see a difference between 720p and 1080p. The human eye simply can't resolve the difference between the two at normal viewing distances.
For example, if you have a 50" set, and you're sitting more than 7-8 feet away (most people would sit more than 10) you will not see ANY DIFFERENCE between 1080 and 720. Your eyes physically can't resolve the difference. It's kind of like taking a wallet-size photo with a 12mp camera and hoping to see a resolution difference between that and a 4mp camera. Ain't going to happen.
This chart might give you a better idea of what I'm talking about:
Bottom line... 90% of customers probably won't see any benefit whatsoever from 1080p. Analyze the chart and make your own assessment. Depending on how close you sit, how big your screen is, etc. you might. If you're in the market for a set, I recommend you go to the store yourself and find two tv sets which are identical models except for resolution. Start at about 25' back, and walk forward until you think you can see a resolution difference. And then, when you think you start seeing a difference, glance at the price tags on the set. 720p sets are usually around 50-70% of the price of their identical 1080p counterparts.
Personally, I did this... and couldn't see a difference (on a 50" set) until around 6' or so. Needless to say, now I have AppleTV hooked up to my 720 Panny Plasma, and it looks GORGEOUS! Absolutely perfect.
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Here are what I consider to be the most important features for image quality on new HDTV sets. 5 and 6 are debatable. you could switch them, I suppose.
(Most Important)
1. High Quality Source
2. Contrast Ratio (brilliant whites, deep blacks)
3. Refresh Rate (LCDs Only)
4. Color Saturation
5. Color Accuracy
6. Resolution
(Least Important)