The old adage in TV retail is that the brightest TV sells. This, in a historical context, makes a lot of sense: the brightest TV caught your eye, seemed "better," and got your hard-earned ducats.
But these days all TVs are bright. And just because one TV is brighter than another, it doesn't mean it's "better."
Heresy, this may seem, but there is some logic behind it. TVs have gotten brighter and brighter over the years. As flat panels developed, the average brightness of your average TV went from "meh" to "ow." Enough so that some people have even noticed their eyes got sore after watching TV.
An analogy can be made with cars. The Lamborghini Murcielago LP 670-4 SuperVeloce has a top speed of 214 mph. The Porsche 911 GT3 has a top speed of 195. Does that make the Lambo a "better" car? Faster, in this limited metric, certainly. But better? TVs, like cars, are more than one single number. Both of these cars are fast. All of today's TVs (well, the ones you'd want to buy) are bright. So to figure out what's best requires digging a little deeper.
More important factors
Brightness/light output is nothing without contrast ratio. Contrast ratio is the difference between the brightest a TV can be, and the darkest. TVs with a low contrast ratio, even if they're bright, will seem washed out compared with TVs with a high contrast ratio.
Here's an image I lifted from my "Contrast ratio (or how every TV manufacturer lies to you)" post, to demonstrate what I mean.
Which image would you rather your TV had? The one on the left, or the one on the right?
Then there's color accuracy. One of the easiest ways a TV or projector company can bump up brightness is by messing with the colors. TVs have gotten a lot better over the years, with many being quite accurate (as in, correctly reproducing the colors that were recorded/filmed by the camera). Projectors, though, often cheat some brightness by using inaccurate colors. This can be the difference between grass looking like grass, and grass looking like a Sour Apple Jolly Rancher.
There's also video processing. Though deinterlacing and scaling have gotten to be quite decent, many new TVs are offering picture enhancement features that occasionally do some good. If one TV seems more detailed than another, wouldn't that be of benefit over raw light output?
Lastly, how much light do you really need? CNET has long held that in most rooms, anything over 40 foot-lamberts is a waste. If you have a bright room and watch TV during the day, maybe some extra light is useful.
But at night, a TV that cranks out 100 fL is going to burn a hole in your skull (figuratively), or at least be a pain in the brain (literally). This is why a backlight control is so crucial on any LCD: extreme light output when you need it, but the ability to turn it down when you don't.
One exception (sort of)
With projectors, brightness is a lot more important. Not only does a projector's brightness determine how bright the image is, but it also only determines the size potential of the image. This isn't to say that the brightest projector is the best-looking, not by a long shot, but it's certainly more of an important factor in the projector's overall performance.
Too often, debates about a particular display's performance seem to get distilled down to a single number. No one number can determine if a TV or projector is better than another, and there are more-important aspects to consider than overall brightness (like contrast ratio, color accuracy, and so on). With TVs, unless the TV is in a really bright room, all will be "bright enough." A better consideration, instead of getting the brightest TV, is darkening the room a bit. If that's not possible, then OK, go for the brightest TV, or at least one with a really good antireflective coating.
Got a question for Geoff? First, check out all the other articles he's written on topics like HDMI cables, LED LCD vs. plasma, active vs. passive 3D, and more. Still have a question? Send him an e-mail! He won't tell you what TV to buy, but he might use your letter in a future article. You can also send him a message on Twitter @TechWriterGeoff or Google+.