I'm gonna guess it's syntax and strings for starters. Erlang makes dealing with strings painful in a lot of ways and while the syntax grew on me to the point where I actually like it now I think most people find it to be painful.
Hrrrmmm as a coder who loves both perl and erlang maybe I'm just permanently warped when it comes to syntax :-)
The author replied, mentioning speed, native executables, syntax and simplicity.
I think he has a point, the only thing I'd disagree with is the syntax.. I don't think it's a major problem, considering you get a functional language with powerful pattern-matching.