In this example:
\documentclass{standalone}
\usepackage{tikz}
\usepackage[utf8]{inputenc}
\begin{document}
\begin{tikzpicture}[xscale=5,yscale=3]
\tikzset{aufgabe/.style={fill=yellow}}
\tikzset{pfeile/.style={->}}
\node [aufgabe] (i1) at (3,10) {Item1};
\node [aufgabe,nearly opaque] (i2) at (4,9) {Item2};
\draw [pfeile] (i1)--(i2) node[sloped,midway,above]{Text};
\end{tikzpicture}
\end{document}
Why does Text not get tilted to the correct angle?

xscale=5,yscale=3. Use the same and it will be OK. – Sigur May 02 '18 at 14:12