The "killer feature" of Gmail that people generally cite is its spam filtering, and that
is hard to replicate, because one reason Google's spam filters are so good is because they have an enormous corpus of messages at hand to train them with. This presents would-be competitors with a chicken-and-egg problem: you need lots of users to be able to pull together a corpus like that, but you can't attract lots of users without your spam filters having that large corpus to learn from.
It's not the interface that's hard to replicate, in other words; it's the backend, the service component. This is pretty consistent with the type of "moats" Google has generally built around its properties, they're almost always more about magic on the backend than magic on the front.
