Trouble combining enumitem and beamer

As you already mentioned, enumitem "disturbs" beamer. This is because the document class (beamer in this case) defines (or redefines) all the necessary commands/macros and environments to it's liking. Then, each loaded package does the same sequentially. As such, since enumitem follows the document class, it redefines the itemize (and other) environments, thereby removing beamer's modified overlay specification.

It is possible to tap into the main level list environment parameters by patching the command \@listI. The following shows the definition of the three levels contained in beamerbaselocalstructure.sty:

%
% List stuff
%

\setlength\leftmargini  {2em}
\setlength\leftmarginii  {2em}
\setlength\leftmarginiii  {2em}
\setlength  \labelsep  {.5em}
\setlength  \labelwidth{\leftmargini}
\addtolength\labelwidth{-\labelsep}

\def\@listi{\leftmargin\leftmargini
            \topsep 3\p@ \@plus2\p@ \@minus2.5\p@
            \parsep 0\p@
            \itemsep3\p@ \@plus2\p@ \@minus3\p@}
\let\@listI\@listi
\def\@listii{\leftmargin\leftmarginii
              \topsep    2\p@ \@plus1\p@ \@minus2\p@
              \parsep    0\p@   \@plus\p@
              \itemsep   \parsep}
\def\@listiii{\leftmargin\leftmarginiii
              \topsep    2\p@ \@plus1\p@ \@minus2\p@
              \parsep    0\p@   \@plus\p@
              \itemsep   \parsep}

A patch using \patchcmd, as provided by etoolbox, that changes \itemsep3\p@ to \itemsep2em (say) modifies the command globally from 3pt to 2em:

\documentclass{beamer}
\usepackage{etoolbox}% http://ctan.org/pkg/etoolbox

\makeatletter
\patchcmd{\@listI}{\itemsep3\p@}{\itemsep2em}{}{}
\makeatother

\begin{document}

\begin{frame}
\begin{itemize}[<+->]
  \item First.
  \item Second.
  \item Third.
\end{itemize}
\end{frame}

\end{document}

The use of the \makeatletter/\makeatother pair is required, since the macro definition of \@listI contains @. Note that this patch does not modify the shrink (3pt) or stretch (2pt) of \itemsep.

You can redefine the \itemize command as defined in beamerbaselocalstructure.sty; in the following example I defined a global itemsep length of 2em (the line signaled with %NEW) for this command:

\documentclass{beamer}

\makeatletter
\renewcommand{\itemize}[1][]{%
  \beamer@ifempty{#1}{}{\def\beamer@defaultospec{#1}}%
  \ifnum \@itemdepth >2\relax\@toodeep\else
    \advance\@itemdepth\@ne
    \beamer@computepref\@itemdepth% sets \beameritemnestingprefix
    \usebeamerfont{itemize/enumerate \beameritemnestingprefix body}%
    \usebeamercolor[fg]{itemize/enumerate \beameritemnestingprefix body}%
    \usebeamertemplate{itemize/enumerate \beameritemnestingprefix body begin}%
    \list
      {\usebeamertemplate{itemize \beameritemnestingprefix item}}
      {\setlength\itemsep{2em}% NEW
        \def\makelabel##1{%
          {%
            \hss\llap{{%
                \usebeamerfont*{itemize \beameritemnestingprefix item}%
                \usebeamercolor[fg]{itemize \beameritemnestingprefix item}##1}}%
          }%
        }%
      }
  \fi%
  \beamer@cramped%
  \raggedright%
  \beamer@firstlineitemizeunskip%
}
\makeatother

\begin{document}

\begin{frame}
\begin{itemize}[<+->]
  \item First.
  \item Second.
  \item Third.
\end{itemize}
\end{frame}

\end{document}

Here's a portion of the resulting third slide:

Adding \tracingall shows that TeX is spinning around a cyclic definition of \\description which is an internal saved version of the description environment. It's quite easy for this to happen if two packages are saving the"original" version of something.

\\description [#1]->\beamer@origdescription 
#1<-

\beamer@origdescription ->\@protected@testopt \description \\description {}

\@protected@testopt #1->\ifx \protect \@typeset@protect \expandafter \@testopt 
\else \@x@protect #1\fi 

\@testopt #1#2->\kernel@ifnextchar [{#1}{#1[{#2}]}
#1<-\\description 
#2<-

\kernel@ifnextchar #1#2#3->\let \reserved@d =#1\def \reserved@a {#2}\def \reser


\@ifnch ->\ifx \@let@token \@sptoken \let \reserved@c \@xifnch \else \ifx \@let

\reserved@c ->\\description [{}]

\\description [#1]->\beamer@origdescription 
#1<-

While it's easy to see why TeX hangs, fixing it depends what you want to do. You could re-assert the beamer or enumitem version. But if you want an environment with the features of both that may take a bit more coordination and knowledge of both packages neither of which I have this morning:-)

for example this keeps beamers defininition and stops the looping.

\let\olddescription\description
\let\oldenddescription\enddescription
\usepackage{enumitem}
\let\description\olddescription
\let\enddescription\oldenddescription