Practical solution for single bold \item in enumerate

An alternative to samcarter's answer is to create a clone of \item which adds the desired formatting:

\def\specialitem{%
  \@inmatherr\item
  \@ifnextchar[%]
    {\@item}%
    {\@noitemargtrue \@item
      [\textbf{\@itemlabel}]% <- The label formatting goes here
    }}

then use as:

\begin{enumerate}
  \item Property 1
  \item Property 2
  \specialitem Property 3
  \item Property 4
\end{enumerate}

MWE:

\documentclass{article}

\makeatletter
\def\specialitem{%
  \@inmatherr\item
  \@ifnextchar[%]
    {\@item}%
    {\@noitemargtrue \@item
      [\textbf{\@itemlabel}]%
    }}
\makeatother

\begin{document}

\begin{enumerate}
  \item Property 1
  \item Property 2
  \specialitem Property 3
  \item Property 4
\end{enumerate}

\end{document}

---

Oh, yes. It's way far from practical :P

You could do something like this:

\documentclass{article}

\begin{document}


\begin{enumerate}
\item Property 1
\item Property 2
\bfseries \item \normalfont Property 3
\item Property 4
\end{enumerate}


\end{document}