Why is there no alternative webmail of the caliber of gmail? Is it a difficult problem? If there were such an alternative, all they would need to do is wait until gmail fouls up, as they did here. Then they'd have a flood of new users. I can imagine a few reasons why there currently isn't an alternative, like network effects stemming from your contact list, or the fact that you'd have to change your email address eve…
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.