Now that I have had a few months to gain experience in mountain biking I would like to offer my observations on skiing vs mountain biking. Maybe others can offer more.
Lots of things transfer well from skiing to mountain biking including some of the feedback. What transfers the best is dynamic movements and the feeling of flow of the center of mass. A quiet upper body and active lower body. Carving transfers well, so does fore/aft movement and flexion and extension. Femur rotation also transfers well as does some countering.
Other things that transfer well are reading terrain and picking line, looking ahead to see where you are going and fast movement over terrain. Some of the DIRT transfers as well.
Skiing moguls transfers nicely to pump with a change in timing. If you know how to use the virtual bump for accelerating in short radius turns on skis then you are set for some real fun pumping turns even without a bump leading in. All one needs is a small berm or a rut.
What is different and what needs some modification. You need to think differently about your feet in relation to the line of force between skiing and mountain biking. If you think of foot position relative to the center of mass and its line through the bottom bracket then you don't have confusion during the transfer. Things like grinding rails and pipe and park help a lot with the foot positions as well. If you ski with static ankles you will have more difficulty with transfer.
Some of the things that need some purposeful practice are that the feet are not flat during fore aft movements but instead rotate around the bottom bracket. One ankle can be planter flexed while another is dorsiflexed. Knee position in relation to the feet in the attack position is also a bit different as the knees tend to be on the same sagital plane.
Your feet are also not in contact with the ground and can range in distance from the ground depending on the turn forces. This takes some getting use to as the amount of angulation can be different than you perceive is necessary at your head level. You have a tendency to move the head too far inside. Having light hands is the key to using the right amount of angulation.
Counter is often misunderstood in skiing and if you misunderstand counter in skiing then counter will not transfer well to mountain biking. If you understand counter for skiing then transfer is a snap. That is especially true if you telemark ski.
Some things that do not transfer well. There is no equivalent for being able to steer the bike in skiing. You have to figure out that steering input changes a lot with the dynamics of the turn. Slower speeds and tighter turns require more steering. Faster speeds or more open turns require less steering. A light touch on the bars is the answer but that is complicated by the fact that arm movements are entirely different than skiing. Getting the inside arm long and at the same time having a light touch and the right amount of bike lean takes some purposeful practice.
More on the arms. The arms can be used for hauling yourself quickly into the front seat or climbing doing wheelies, manuals, jumps and hops. The trick is to keep the rest of the upper body, especially your head quiet and get the arms right. Again more practice.
One negative transfer with the arms is missing the tree on the inside of the turn. If you are use to skiing trees fast then you really don't look at the trees. When hauling on skis though trees you sub-consciously move your inside hand in to miss the tree just like in gates. The problem with the mountain bike is the damned handle bar is still out there and can clobber the tree. It took me three times and some reflection while on the ground to learn how to pay enough attention to pick a different line and entry speed.
Gear shifting and using the brake have little transfer so they take some purposeful practice to gain reasonable confidence to do it automatically.
Pedalling and spining take some purposeful practice. I did a lot of that on a spining bike and then worked my way up to doing it while in the attack position.
Climbing has some transfer if you have skinned. I find that I like climbing a whole lot more than I thought I would. Especially if its tight or techical. Getting the strategy and conservation of momentum down through sprints and spins takes some practice and a few dismounts.
I now stack up pretty good against the locals who run the trails that I am runing. I pass far more frequently than I am passed. There are a few riders in these parts that blow me out of the water and make your you know what feel like the size of a crayon with two peas.
So what am I still struggling with. What else, the wheelie and the manual. I can easily lift the back of the bike and or load the entire bike and get it off the ground, even on flat pedals. The problem I am having with the wheelie and manual is my hands. I have Raynaud's and trigger finger along with a thumb disorder (can't remember the syndrome name). It is bad enough that using straight arms and applying back pressure creates a situation where I have no sense of position or balance through my hands. I can get the bike up almost to the balance point through pedalling but from that point to landing on my tailbone is not much more. With the hand problems I have no sense of feathering the brake either. Its either not quite up or loopout without being able to use my arms or hands to get off the saddle and land on my feet. Three times on my ass in the back yard is enough. My buddy learned how to wheelie on the stingray with wheelie bars back in the 60's. I think I will make a set of narrow wheelie bars that will let me go just past the balance point to where I can learn how to detect it through my hands and have some use of the brake. I think if I can learn to find the balance point and control side balance without using my hands to bring the bike up I will have some use of the rear brake. Wish I had learned to do that at 10 years old instead of 57.
Keeping a manual up is even scarrier. I can easily load the bike shove with my feet shove/pull with the arms/hands and loop out fast. Not so good in the woods full of trees and posion ivy.
Bunny hops don't seem to be near the problem of sustained wheelie or manual. I don't have that fear of possibly looping out in a bad spot. All I have to do is look at the obstacle and fill in the spaces in front and back with virtual dirt. I then treat it like I am pumping a bump. Load the bike and away we go, front wheel first. Loads of fun. Especially when you add a little twist to land lined up with a turn on the other side. Now for double obstacles, I need that damned reliable manual.
I have not gone over the handlebars yet. I tend to be very balanced over the bottom bracket with a light hand touch. Pumping everything that will pump is another thing that keeps me in the cockpit. Having bad hands that scream with much of a grip is a sure reminder of a light touch.