Your contact points and your finishing kit. Bars, stems, headsets, pedals, seat posts, grips, saddles and that sort of stuff.
The finishing kit on your mountain bike is where you get the opportunity to be fine tune and be creative with your bike build. Some components have more of an influence on a bike handles than others but it's still surprising how the small things can end up dominating your experience if they're not quite right. Stems, bars and saddles are the obvious items that are important to get right. But an uncomfotable pair of grips or a rattly pair of pedals are hard to ignore. Some things are more important than others but everything is important in its own way.