Mine weighs around 20lb, but its not got amazing gear on it so could be lighter if i thew money at it.
Geometry wise more similar to a road bike than mtb, angles are a bit slacker and rear end has much more clarence for tyres and mud. The frames are a but stronger than a road frame too, and have canti bosses ofcourse.
cassette can be either mtb or road(if you use shimano ), but normally road and a road chainset with smaller rings, 39 / 44 is typical. All other gear its usually road too, even wheels. There are some specific cross parts out there but most folks just use road bits
I have seen retro mtbs used with drop bars, but they can't keep up
So really they are nothing like an mtb! More like a beefy road bike