Lisp is the hardly the simplest language. It has way less 'stuff' to learn, but that 'stuff' in imperative languages is generally useful abstractions that you don't to learn, but instead simply know of. By that measurement, x86 Assembly is really the simplest language of them all, but I don't think I need to explain why that's equally absurd.

Lisp is a difficult language due to it's density and unreadability.

Useful stuff hasn't been built in functional languages until very recently, exception being some AI. Haskell and friends are making functional languages cool and useful (and that's great!) but if you want to build something useful as soon as possible, learn .NET, Java, Python or Ruby.