The strong point of the movie, and one of Kubrick's best talent, is the atmosphere. You get sucked into it by the time the couple settle in as the guardians of the lodge.
The young boy really is the meat to the horror of it all, he's the one to give you the chills, helped with some very smart editing and a deeply persuasive soundtrack. The music is great. Jack Nicholson on the other hand is the one to make us afraid with his excellently well played psychotic behavior. Shelley Duvall is the born victim, excusing her husband, bending to his will, doing his job, mostly clumsy, rather clingy, a bit clueless... it's all in her acting, and it's spot on.
The film is obviously getting old, but you really rarely take stock of it, like the awkward generic, and a few lousy sound pickups in the beginning. I'll relegate the general stiffness of the introduction, until they are alone at the hotel, as building up the atmosphere.
Overall, if you haven't seen it yet, you should.