In fact I can't see much wrong with it. The performances are great. I'd loved to have seen James Mason as Komarovsky (he turned it down) which isn't to say that Rod Steiger isn't more than adequate. Chaplin is sweet, Sharif is soulful, Christie does a wonderful job of turning a dream girl into something real, Guinness is miscast but has his moments and Richardson is a joy. Klaus Kinski puts in a characteristic appearance being dangerous and mad. Russia (actually Spain and Finland and other far-flung places) looks beautiful.
I don't suppose there's any doubt now that Lean is one of the great directors.
I read the book around the time I saw the movie- in a cheap paperback edition with Sharif and Christie and a steam train on the cover- and found it a slog- an attempt to do something Tolstoyan by a writer who wasn't really a novelist. The cheerleaders of the west wanted it to be an enduring classic because it was a wet fish to slap the Soviets with- but I doubt they were right. Do people still read it? I suppose it's of historic interest and importance. Am I going to re-read it? No. But I'd happily watch the film again.