Every single non-standard extension, everything not in the spec, is "wrong" with Common Lisp. This includes any support for threads, filesystem access, processes and IPC, operating system interoperability, a GUI, Unicode, and the long list of other features missing from the latest hyperspec.Point taken, to some degree. But that problem doesn't directly affect you if you're an architect building an application or web service -- you just pick Allegro or LispWorks or CMUCL or SBCL or CLISP, and you're done. Each Lisp has its own APIs for those services. It's true your code won't port trivially from one platform to another. That's usually not much of a concern, if the platform you choose runs on all the hardware and OSs you need.
But the balkanization indirectly affects you. It impedes the development and distribution of free libraries. If you're like me, you've become addicted to free libraries. A jabber component, or a message queue, or task scheduler, is just a Google away, if you're developing in Java.
To build a cross platform library, a Lisp developer has a fair amount of work to do. At a minimum, if you use any extensions, like sockets or Gray streams, you have to find or write an abstraction that may do little more than hide the package name of your implementation. You may not be able to abstract away some issues -- does each sockets implementation know how to send out of band TCP (MSG_OOB)? Which methods do the underlying Gray streams really require you to override?
Common Lisp has been frozen since 1994, and as best I can tell, there is no process by which Gray streams, sockets, or IPC, or anything could be added to the language now. Contrast to Java Community Process, which for whatever warts, has enabled the language to evolve uniformly across implementations. You never have to worry that your Java 5 code that you developed using Sun's JDK won't run on Jrocket or IBM. It just works.
Lisp needs some JSR-like, ongoing process -- not another eight year, big bang ANSI revision -- to standardize changes we take for granted in all other environments. And a benevolent dictator would help.
 How ironic that Peter Seibel contrasts the long JSR process against the capability Lisp gives you to extend the language with macros.