what's missing?

According to the IMDB, the following were cut out when the film was purchased by MGM in 1941: "Originally released at 97 minutes. Later reissues are taken from a shortened 82-minutes print. Deletions include:

A 3.5 minute segment immediately following the opening credits. This is filmed in first person, and shows Jekyll playing the organ and getting ready for his lecture.
Jekyll helping a young girl to learn to walk in the free ward. This 1 minute scene preceeds the scene with the sick woman in bed.
After his first transformation, Jekyll does not go immediately to the pub as in the cut version. Instead, Poole comes to the laboratory, and Jekyll takes the antidote and then lets him in. Jekyll then visits Muriel and learns that she is going away on a trip. Jekyll is preoccupied with her absence. When he learns she will be away another month, Poole suggests he go out. Jekyll knows a man of his position cannot be seen in the establishments of the lower classes, so he decides to take the potion again. Another on screen transformation occurs, this time while he is seated in a chair. He then leaves for the pub at which Ivy is singing. This sequence lasts 6.5 minutes.
Just before Jekyll's transformation in the park, the restored scene reveals the reason for his transformation without taking the potion. He sees a bird being killed by a cat up in a tree. The traces of the drug in his combined with the witnessing of this violent act is enough to trigger the transformation, which he now has no control over. This restored cut lasts 45 seconds.
The last restored scene is when Jekyll visits Muriel to "set her free". This adds additional details as to the torment Jekyll is going through, and confusion of Muriel as to what is troubling Jekyll."

The early 1990's VHS copy restored all of the above. The 2003 DVD restored a nude scene featuring Miriam Hopkins, as well. However, there are still photographs that contain evidence that more scenes were cut, and are now considered "lost." These include Mr. Hyde running through the streets, and knocking into a boy, and then stepping on his face, Mr. Hyde biting into a live bird, and eating it, and Mr. Hyde whipping one of the two female characters.

Hope this helps!

Cheers!

Sincerely, Steve B