I often see って being used to mark a topic, such as in the sentence 新宿ってどこ？ I'm aware that this is more colloquial compared to the topic marker は. Other than the formality aspect, are って and は completely interchangeable in meaning when used like this?
は is fairly matter of fact. "Where is Shinjuku?"
って is a little more nuanced. Its like "Oh, now that you mention Shinjuku...where is it?" or "Speaking of Shinjuku, where is that?"
For all intents and purposes I gather the actual end-point meaning is the same but って is linking it more with something that has been previously said whilst は could just be bringing it up out of the blue. I feel that because it has this prior link it comes across as less forceful, just like such a situation in English.
A Dictionary of Basic Japanese Grammar has the following to say about this:
When って is attached to a noun, it is close in meaning to the topic marker は。When って is attached to a sentence as in
it is closer in meaning to 「..というのは、..」. However, it is more colloquial and emotive than は and というのは。In fact, if the predicate does not express the speaker's emotive judgement / evaluation, って cannot be used. For instance, the following is ungrammatical:
The following use is correct
There are also a few more examples in that section which use the って construction with nouns: