I'd rephrase that^. soft flex is pretty much used only for park. It gives you the ability to tweak your board more. Medium flex gives you more support for all mountain, but I personally feel it's still great if used in the park. stiff flex is pretty much a mountain oriented boot. If you're not riding park and you want as much response/as little flex as possible, you're gonna want to go with that. I personally suggest medium flex. It will suit most riding styles and will definitely keep you happy.
When it comes to bindings, it's pretty similar to boots. I run a "jibbing" binding with medium flex boots (burton cobrasharks/nike zoom force 1) so I have flex when I really need it but support when I don't. I feel that sticking to only stiff flex or only medium flex or whatever, is not giving you a lot of freedom.
For a board, I think that is really up to you. I think shopping around for a board, taking your time reading reviews, and seeing what fits your style and interests is the best way to gauge what type of board you're looking for. Remember, don't worry about the brand name, just try to find what fits what you want to ride.