only.tex

\documentclass{beamer}
% http://tex.stackexchange.com/q/217628/1402

\begin{document}
\begin{frame}{Uncover}
\begin{itemize}
\item<1> a really long item, which takes up almost the entire page
\item<2> another item
\item<3> yet another one
\end{itemize}
\end{frame}

\begin{frame}{Only}
\begin{itemize}[<only@+>]
\item a really long item, which takes up almost the entire page
\item another item
\item yet another one
\end{itemize}
\end{frame}

\end{document}