I've commented before on the fact that My Fair Lady is a direct adaptation of the 1938 movie version of Pygmalion. Alan Lerner initially downplayed this and talked as if the additional scenes were his own idea, but eventually he wrote that the key to adapting Pygmalion was "following the movie more than the play." Other people had tried to adapt Pygmalion and, apparently, focused too much on Shaw's original play; by incorporating the movie's extra scenes, the ending (which Shaw didn't write) and the mid-point crisis from the movie (the Professor Karpathy scenes, which provided the story with an actual villain), Lerner came up with a viable musical. It wasn't the first show to succeed by leaning heavily on the movie version as a source; The King and I is basically a direct adaptation of the script for the movie Anna and the King of Siam, but the writers of that movie didn't get any credit anywhere in the show (kind of shabby, but not unusual for Rodgers and Hammerstein, who didn't like sharing credit).
Anyway, I strung together side-by-side comparisons of three scenes from Pygmalion with their counterparts in My Fair Lady: the marbles gag, Karpathy's identification of Eliza as a "fraud," and the ending. The biggest difference, obviously, is that everything is bigger and longer and slower in My Fair Lady. Pygmalion is a very fast-paced movie (only 99 minutes), and it was done as a contemporary 1938 piece -- which may not have been the best choice, but it probably wouldn't have been in the budget to do it as a period piece. My Fair Lady was always supposed to be a lavish-looking period piece; even though the original Broadway production actually had a rather modest budget, director Moss Hart and designer Oliver Smith made it look sumptuous and huge, and of course the movie version was a big-budget affair all the way. So the marbles gag is twice as long in My Fair Lady as it was in Pygmalion, and even though the ending is exactly the same, it's stretched out much more.
The most interesting comparison is between the Karpathy "revelation" in Pygmalion and in My Fair Lady. In the new scene written by Shaw for Pygmalion, Karpathy explains that Eliza can't be English because "only those who have been taught to speak it, speak it well." In My Fair Lady, the scene ends before this (I believe that in the original production they actually had the act break here) and Higgins tells us what happened in the "You Did It" number. This is a case where the musical improves on the original movie -- adding some extra suspense and giving Eliza a chance to actually be there when this is discussed.