Global setting of spacing between items in itemize environment for beamer

In most documents, you can do this, which avoids the use of extra packages.

\let\tempone\itemize
\let\temptwo\enditemize
\renewenvironment{itemize}{\tempone\addtolength{\itemsep}{0.5\baselineskip}}{\temptwo}

(Stufazi suggested a neater way of doing this in his answer, which I will use below.)

However, I think that the frame environment in beamer resets the properties of itemize. You could do something like this, but it will prevent frame's optional arguments from working.

\documentclass{beamer}
\let\oldframe\frame
\renewcommand{\frame}{%
\oldframe
\let\olditemize\itemize
\renewcommand\itemize{\olditemize\addtolength{\itemsep}{100pt}}%
}
%
\begin{document}
\begin{frame}
\begin{itemize}
\item The first.
\item The second.
\item The third.
\end{itemize}
\end{frame}
%
\begin{frame}
\begin{itemize}
\item The fourth.
\item The fifth.
\item The sixth.
\end{itemize}
\end{frame}
%
\end{document}

Alternatively, you could try this, but I can't guarantee that it won't break something else.

\documentclass{beamer}
\newlength{\wideitemsep}
\setlength{\wideitemsep}{\itemsep}
\addtolength{\wideitemsep}{100pt}
\let\olditem\item
\renewcommand{\item}{\setlength{\itemsep}{\wideitemsep}\olditem}
%
\begin{document}
\begin{frame}
\begin{itemize}
\item The first.
\item<2-> The second.
\item<3-> The third.
\end{itemize}
\end{frame}
%
\begin{frame}[shrink=50]
\begin{itemize}
\item The fourth.
\item The fifth.
\item The sixth.
\end{itemize}
\end{frame}
%
\end{document}

It might be safer to define your own list environment based on itemize and use this in future; thus

\newenvironment{wideitemize}{\itemize\addtolength{\itemsep}{100pt}}{\enditemize}

This would avoid the necessity for hacks that have unwanted side effects.

Just put the two line in your foredocument (change the 100pt to any value you want):

\let\OLDitemize\itemize
\renewcommand\itemize{\OLDitemize\addtolength{\itemsep}{100pt}}

And if next time you write your document using the enumitem package (recommended!), you could

\usepackage{enumitem}
\setitemize{itemsep=100pt}

After diving a bit into beamer's source files, I think that I've found the best way to implement this without breaking other options or advanced features of the itemize or frame environments. The solution is to define a new frame option like:

\makeatletter
\define@key{beamerframe}{wide}[30pt]{%
  \def\beamer@cramped{\itemsep #1\topsep0.5pt\relax}}
\makeatother

So that one can later write:

\begin{frame}[wide]
% ...
\end{frame}

to get “wider”, more spacey, versions of all the itemize, enumerate and description environments used in the frame. This is actually just a copy from another squeeze option already defined by beamer, so I'm guessing this would be the “proper” way to implement this without breaking anything.

---

Set as default

One would also want, probably, an option to make this the default for all frames. Unfortunately beamer does not seem to support setting default frame options. The following is a bit of a hack, but I'm hoping it not to break anything. Throw all of this code in a style file (say beamer-extra.sty):

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{beamer-extra}[2013/08/22 Some extra beamer features]
\RequirePackage{etoolbox}

% A command to set default frame options:
%   \defaultframeoptions{<options>}  % append some more defaults options
%   \defaultframeoptions*{<options>} % clear past defaults, set some new
\def\beamer@extra@frameoptions{}
\def\beamer@extra@setframeoptions#1%
  {\appto\beamer@extra@frameoptions{\setkeys{beamerframe}{#1}}}
\def\defaultframeoptions{\@ifstar
  {\def\beamer@extra@frameoptions{}\beamer@extra@setframeoptions}
  {\beamer@extra@setframeoptions}}
\define@key{beamerframe}{environment}%
  {\def\beamer@frameenvironmentsubst{#1}\beamer@extra@frameoptions}

% A new frame option for wider items
\define@key{beamerframe}{wide}[30pt]{%
  \def\beamer@cramped{\itemsep #1\topsep0.5pt\relax}}

And then use it like:

\documentclass{beamer}
\usepackage{beamer-extra}
\defaultframeoptions{wide=15pt}

\begin{document}
\begin{frame}
\frametitle{Example}
\begin{itemize}
\item One
\item Two
\item Three
\end{itemize}
\end{frame}
\end{document}