I didn't bring up Casablanca originally, CogSmooch did:
http://www.smashboards.com/showpost.php?p=10087133&postcount=76465
but I ran with it because I think it proves my point pretty well.
As to my two statements, they're not incompatible, they go together.
A film can't be appreciated without its context. So if you view an older film without its context, it doesn't stand up well to current films. We're steeped in the current context, whether we realize it or not. We're not necessarily steeped in World War II-era North Africa (Casablanca) or Great Depression/Dustbowl era Kansas (The Wizard of Oz). What I'm saying is that if you do not know those things about those two movies (that is, the context they are set in), then it becomes much more difficult to understand them and compare them favorably to contemporary movies, which we understand by default. I
strongly believe that context is necessary to understand art. CogSmooch has said that he does not, and that's where the question about Casablanca comes from.
Mogwai's example was the case study of this: viewing Akira without the context of what made it special in its day reduced it to an average movie in comparison to a modern classic like Spirited Away (if I'm putting words in your mouth Mogwai, please correct me).
EDIT- I should say also that I'm assuming in this conversation that when we use the word "appreciate" or any synonym thereof, we mean more than just "liking" the film. Sure, you can like a film without understanding it. But to "appreciate" it (that is, engage with it on a more intellectual level), I think cotextual understanding is crucial.