Here is a quite automated solution:
\documentclass{article}
\usepackage{longtable}
\usepackage{etoolbox,array}
\preto\tabular{\setcounter{magicrownumbers}{0}}
\newcounter{magicrownumbers}
\newcommand\rownumber{\stepcounter{magicrownumbers}\arabic{magicrownumbers}}
\usepackage{enumitem}
\newlist{tabenumi}{enumerate}{2}
\setlist[tabenumi,1]{label={\themagicrownumbers.\arabic*},nosep,leftmargin=*,before=\vspace{-\baselineskip},after=\vspace{-\baselineskip}}
\setlist[tabenumi,2]{label={\textbullet},nosep,leftmargin=*}
\newlist{tabitemize}{itemize}{1}
\setlist[tabitemize]{label=\textbullet,nosep,leftmargin=*,after=\vspace{-\baselineskip}}
\usepackage{calc}
\newlength{\mycolumnwidth}
\setlength{\mycolumnwidth}{\textwidth-4\tabcolsep-4\arrayrulewidth-2em}
\begin{document}
\begin{longtable}{|@{\makebox[2em][l]{\space\rownumber.}} |p{0.5\mycolumnwidth} |p{0.5\mycolumnwidth}|}
\hline
Some prety long text including itemize
\begin{tabitemize}
\item an item
\item another item
\end{tabitemize}
& Another long text that can contain linebreaks\\
\hline
\begin{tabenumi}[series=test]
\item item
\begin{tabenumi}
\item item
\item item
\end{tabenumi}
\item item Additional text
\begin{tabenumi}
\item item
\item item
\end{tabenumi}
\end{tabenumi}
& Additional text to 2.1 \vspace{2\baselineskip}\newline Additional text to 2.2\\
\hline
\end{longtable}
\end{document}

The row numbers in the table are generated automatically. The code for this is borrowed from here and slightly modified.
I have used the enumitem package in order to create two new enumerate- and itemize-like lists for the use in table cells. The first level of tabenumi behaves like an enumerate environment and automatically adds the rownumber (left column) to its numbering scheme. The second level of tabenumi behaves like an itemize environment and uses a \textbullet insetad of a number.
The tabitemize environment is intended to use without a surrounding tabenumi environment as shown in the first row of the table.
Lastly, I have defined a new length in oder to calculate the column widths that are necessary to make the table as wide as the textwidth.
For a more open look of the table, you might also want to consider deleting all vertical lines and replace the \hlines by horizontal rules from the booktabs package. In the following example, I have marked all the lines with changes compared to the first example, with %<----------.
\documentclass{article}
\usepackage{longtable}
\usepackage{etoolbox,array}
\preto\tabular{\setcounter{magicrownumbers}{0}}
\newcounter{magicrownumbers}
\newcommand\rownumber{\stepcounter{magicrownumbers}\arabic{magicrownumbers}}
\usepackage{enumitem}
\newlist{tabenumi}{enumerate}{2}
\setlist[tabenumi,1]{label={\themagicrownumbers.\arabic*},nosep,leftmargin=*,before=\vspace{-\baselineskip},after=\vspace{-\baselineskip}}
\setlist[tabenumi,2]{label={\textbullet},nosep,leftmargin=*}
\newlist{tabitemize}{itemize}{1}
\setlist[tabitemize]{label=\textbullet,nosep,leftmargin=*,after=\vspace{-\baselineskip}}
\usepackage{calc}
\newlength{\mycolumnwidth}
\setlength{\mycolumnwidth}{\textwidth-4\tabcolsep-2em} %<----------
\usepackage{booktabs} %<----------
\begin{document}
\begin{longtable}{@{\makebox[2em][l]{\space\rownumber.}} p{0.5\mycolumnwidth} p{0.5\mycolumnwidth}} %<----------
\toprule %<----------
Some prety long text including itemize
\begin{tabitemize}
\item an item
\item another item
\end{tabitemize}
& Another long text that can contain linebreaks\\
\midrule %<----------
\begin{tabenumi}[series=test]
\item item
\begin{tabenumi}
\item item
\item item
\end{tabenumi}
\item item Additional text
\begin{tabenumi}
\item item
\item item
\end{tabenumi}
\end{tabenumi}
& Additional text to 2.1 \vspace{2\baselineskip}\newline Additional text to 2.2\\
\bottomrule %<----------
\end{longtable}
\end{document}
