I don't think that the iPad 3 is less bright than previous versions... to me it seems more bright actually. What I have noticed is the slider sensitivity has changed. It allows you to have much more control in the dimmer range, below the half-way point. So if the slider is at 50%, the iPad 3 is definitely dimmer than the iPad 2 is at half slider, but when you move the slider past 50%, the brightness increases at a faster rate, up to or past the iPad 2 brightness. So it's like the slider is not to scale, where the slider from 0 - 50% is like brightness from 0 - 25%, whereas slider from 50% - 100% is like brightness from 25% - 100%. I'm having trouble explaining what I mean - does that make sense?
I like the iPad 3 brightness control more - it allows you to tweak the brightness a lot more when using the iPad in the dark. I always found it hard to find the brightness sweet spot for night use with the iPad or iPad 2, but have no problem with the iPad 3.