Borrowing from the page of Gonzalo at LaTeX Beamer: Define \itemsep globally, one can redefine indentation parameters like \leftmargin and \itemindent by patching the definition of \itemize.
EDIT: I also show another way with stacks.
\documentclass{article}
\usepackage{xpatch,lipsum}
\usepackage[usestackEOL]{stackengine}
\def\defaultitemindent{0pt}
\def\defaultleftmargin{\the\leftmargin}
\xpatchcmd{\itemize}
{\def\makelabel}
{\setlength{\itemindent}{\defaultitemindent}%
\setlength{\leftmargin}{\defaultleftmargin}%
\setlength{\topsep}{0pt}\def\makelabel}
{}
{}
\begin{document}
\parindent 0pt
DEFAULT:
\begin{itemize}
\item[Really Long Text] \lipsum[4]
\item[Short] text
\end{itemize}
STACK:
\begin{itemize}
\item[\smash{\Longunderstack[l]{Really\\ Long\\ Text}}] \lipsum[4]
\item[Short] text
\end{itemize}
CHANGE ITEMINDENT
\def\defaultitemindent{15ex}
\begin{itemize}
\item[Really Long Text] \lipsum[4]
\item[Short] text
\end{itemize}
CHANGE LEFTMARGIN
\def\defaultleftmargin{20ex}
\def\defaultitemindent{0ex}
\begin{itemize}
\item[Really Long Text] \lipsum[4]
\item[Short] text
\end{itemize}
\lipsum[4]
\end{document}

descriptionorlabeling(KOMA-script)? It isn't recommended at all to use long texts there. You could even be on the search forparagraphorminisec(KOMA-script). Can you show us a compilable real world example? – Johannes_B Mar 02 '15 at 21:11\\(Latex will be complaining about underfull hbox of badness 10000 (which is infinitely bad) – David Carlisle Mar 02 '15 at 21:52