[haspok]

There is an important change in the angle in your hip, so leg vs torso. Modern MTB geometry puts you in a more upright position, and that opens the hip angle. This means for climbing you use more of your glutes and less your quadriceps. This is great, because glutes are the ultimate athletic muscle.

Of course, you have to get used to this position, which is markedly different to the road bike / track bike leaning ahead and closing your hip position. It also gives you great front-back balance and easier control on your bike. Easier to jump (or just shift weight), and you have the wide bars for greater leverage.

Seriously, if someone has not tried a MTB with a modern geometry, they definitely should, they will be blown away by how much better it is than the old ages. Add to this the dropper seatpost, and you have a winner!