To add to the discussion, when talking about board flex, there are two components that make up the board`s overall flex. You have longitudinal flex which describes the flex of the board from tip to tail or how easy it is to bend the board. The other flex is called torsional flex. This flex describes how easy or resistant the board is to twisting. For the new rider, learning to manipulate their board, torsional flex is most important.
As you learn to snowboard, you will be using your feet to twist the board to engage the sidecut to initiate turns. A soft board makes responds to this twist much more and is actually much more responsive at slow speeds. A stiffer deck requires more confident, aggressive torsional movements and is less responsive at slow speeds. As speed increases, the stiffer deck becomes more responsive but remains stable. The softer board does not really become less responsive at high speed, but becomes unstable. This is why your first choice for high speed carving is not going to be a noodly jib board.
This is where the all mountain intermediate board shines. It is soft enough for a beginner to learn easily on, yet is stiff enough to handle faster speeds and more advance riding techniques. In my experience a board with a mid to upper mid flex rating is ideal as a first purchase since it fills this demand nicely. Another great thing about the technological advances in modern boards is that with all of the space age materials companies are putting in their boards, it is possible now to have a longitudinally still board that is torsionally soft.
You asked about sidecut and its effect on beginner versus intermediate or advanced riding. The deeper the sidecut, the quicker the board will enter, negotiate and exit a turn. Mellow sidecuts tend to ease into and out of turns and are ideal for both beginners to learn on as well as high speed carving where the rider does not want to be thrown into and out of turns. On the flipside, a deeper sidecut is ideal for the rider who wants a very responsive, quick turning board for riding steeps and trees for example. Sidecuts come in many flavors too; you have your basic radial sidecut that is even throughout the length of the board. Then you have more specialized sidecuts like progressive sidecuts that the actual radius of the sidecut changes along the edge of the board. These different designs allow the board to enter and exit turns differently. A great example of a progressive sidecut that a beginner would appreciate is one where the beginning of the sidecut is very mellow to make turn initiation gentle, then it deepens to make the board turn very aggressively once on edge and into the turn.
When it comes to riding, it is true that it is "the Indian not the arrow" when it comes to equipment versus skill, but today`s equipment can make a huge difference especially for the new rider...