Outlook.com's spam filtering is at least as good as Gmail's now, probably better. The only significant difference is that it's much less aggressive. Outlook.com will let two or three spams through where Gmail will put four or five legitimate emails in the spam box.
Since most people probably don't check their Gmail spam boxes regularly, Gmail's spam blocking is actually more dangerous.
To be fair, I get a lot of press releases, newsletters and bulk mailings. But there's really no excuse for Gmail to keep putting emails from, say, EE Press Office in spam when I take them out every time. (My account was an early beta.)
Also they seem to engage in very spammer-friendly activities such as notifying spammers if and when you read their spam email (if images are turned on by default). Soooo, I can't imagine that competing against their spam filtering is as impossible as people tend to believe.
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.