Indeed, "melting pot" makes me think of think-tanks or a busy "scene" area, it doesn't make me think of an actual melting pot, the words themselves doesn't conjure up the mental image that they refer to, so the strength of the metaphor is weak.

Reading Orwell's own writing in this article "like tea leaves caught in the sink" it conjures up a fresh mental image.

One thing I discovered during 6 years of living in central Europe is that speaking mostly with non-native English speakers leads to a necessary elimination of metaphors; you have to use direct words. I can't say "he's trying to pull the wool over your eyes" or "don't let the cat out of the bag" or whatever, because nobody has any idea what I'm saying, and it's not worth explaining. I considered it a kind of mental training, it taught me to speak plainer English.