It's hard (if not impossible) to pick a single attribute of 'Lisp' that's shared by all the languages that are part of the Lisp family of languages. There are Lisps without macros, there are Lisps without the simple syntax, there are Lisps without dynamic typing, or lexical scope, or any of a dozen other features that are present in at least on Lisp.... so I think reducing Lisp to a handful of features and calling that 'basically it' is a good way to miss the point. (Not to belabor the obvious

One of my favorite points about Lisp was made by Kent Pitman back in 1995: http://www.nhplace.com/kent/PS/Lambda.html

In it, he describes through a though experiment what it means for a language to be defined by its community and how that impacts its development and use. It isn't as much about the specific features as it is the approach that led the language to get to have those features in the first place.

Just to illustrate, the simple syntax is a good example. It's not immediately obvious why simple syntax is a good thing... in fact, given the fact people prefer infix and a big part of the backlash against Lisp is due to the syntax, you might argue that it'd be better for Lisp to drop the syntax entirely. (As was tried in Dylan and Logo) That, you'd lose a lot that's not necessarily obvious. There's a real value in having a concrete syntax for your language's core data types and then defining the language in terms of those data types. The simple syntax isn't the point as much as what it enables you do accomplish.