Under dark conditions, the eye's pupil dilates to around 7 mm. So to maximize the amount of light a pair of binocs can stuff into your eyes, you need an exit pupil also around 7 mm. You can work this out for binocs by dividing the object lens diameter by the magnification. So in the case of 8 x 42 (above), the exit pupil is 5.25 mm. A pair of 6 x 42 binocs (if they existed) would give you a 77% brighter image. The calculation works consistently across all magnifications and apertures. So 7 x 50 would have the same brightness, as would 3 x 24. Any exit pupil bigger that 7 mm is wasting gathered light, because it doesn't get into the eye.
All things not being equal, right now you should always compare the binocs you lust after with image-stabilized Canons: the stabilization makes a huge difference in the perceived performance because the image isn't jiggling around, and they're good value. My guess is they add around 30% to the apparent brightness for a given aperture (say 2 stops if you're a camera person) since the eye integrates light quite well. I'm not saying "buy Canon", I'm saying use them as the yardstick and see if you can beat them at a price point you're happy with.
Also consider eye relief if you wear glasses: it's the plane the image is formed on. Binocs with extended eye relief will work better through spectacles than those with short eye relief, though short eye relief can give a more "immersive" experience, where you appear to be able to look around inside the image without moving the binocs.