Check out the following example. I would like to write the code as in the second list, but get output equivalent to the other two. However, there is a lot of extra vertical space after the third item of the second list, and I don't know why, or what to do about it.
\documentclass{article}
\newenvironment{listitem}{\item}{}
\begin{document}
\begin{enumerate}
\item
Lorem ipsum dolor sit amet, consectetur adipiscing elit.
\item
Lorem ipsum dolor sit amet, consectetur adipiscing elit.
\item
Lorem ipsum dolor sit amet, consectetur adipiscing elit.
\[
2 + 2 = 4
\]
\item
Lorem ipsum dolor sit amet, consectetur adipiscing elit.
\end{enumerate}
\hrule
\begin{enumerate}
\begin{listitem}
Lorem ipsum dolor sit amet, consectetur adipiscing elit.
\end{listitem}
\begin{listitem}
Lorem ipsum dolor sit amet, consectetur adipiscing elit.
\end{listitem}
\begin{listitem}
Lorem ipsum dolor sit amet, consectetur adipiscing elit.
\[
2 + 2 = 4
\]
\end{listitem}
\begin{listitem}
Lorem ipsum dolor sit amet, consectetur adipiscing elit.
\end{listitem}
\end{enumerate}
\hrule
\begin{enumerate}
\begin{listitem}
Lorem ipsum dolor sit amet, consectetur adipiscing elit.
\end{listitem}
\begin{listitem}
Lorem ipsum dolor sit amet, consectetur adipiscing elit.
\end{listitem}
\begin{listitem}
Lorem ipsum dolor sit amet, consectetur adipiscing elit.
\[
2 + 2 = 4
\]
\end{listitem}
%
\begin{listitem}
Lorem ipsum dolor sit amet, consectetur adipiscing elit.
\end{listitem}
\end{enumerate}
\end{document}
