The sets are breathtakingly beautiful. Whether the cast is on an archaeological dig in the dessert, in a flashback to ancient Egypt, or in a stately English manor, every set gets the royal treatment. I've always loved Hammer Studios for it cinematography and set design. The Mummy represents some of their best work in this area.
Furthermore, Lee and Cushing are in top form, giving some of the best performances of their careers. Lee is particularly strong as Kharis. His make up is flawless and the way he physically inhabits the role is subtle, strong and believable. Cushing is also great as the archaeologist. It's the kind of role he was born to play and he brings some nice moments to the screen as well. The rest of the cast is excellent and there is really not a weak actor in the bunch.
Kudos to director Terence Fisher [Horror of Dracula, The Curse of Frankenstein] for giving us such a beautifully made and well told story. A total shout out to make up artist Roy Ashton for helping to bring to life a mummy that is worthy of the work of legendary make-up artist Jack Pierce. Don't miss this one.
For more info check out the film's entry in IMDB.