An example of a table with lots of whitespace.

table-example.tex

\documentclass[article,a4paper,12pt,twoside]{memoir}

\usepackage{hyphenat}

\setlrmarginsandblock{40mm}{20mm}{*}
\setulmarginsandblock{20mm}{20mm}{*}

\begin{document}

\renewcommand{\arraystretch}{1.5}
\begin{table}
\begin{center}
\raisebox{3in}{\small
\begin{tabular}{|p{.25\textwidth}|p{.25\textwidth}|p{.25\textwidth}|}
\hline 
% \multicolumn{1}{p{.25\textwidth}}{\textbf{Relevance}}
\begin{center}(PM)
\end{center}

Ultimately relevance depends on peer review, and irrelevant content
may be deleted.  Mechanisms to ensure that relevant content
\emph{will} be added could be improved. &
\begin{center}(WP)
\end{center}
 People contribute articles about what
they're interested in; apart from this, rules like WP:WEIGHT come into
play.& 
\begin{center}(DO)
\end{center}

Anyone can upload projects (for ``full projects'', one time approval
is needed), but getting changes into the core requires considerably
more vetting. \\
%\multicolumn{1}{p{.25\textwidth}}{\textbf{Quality}}
Quality control is handled with corrections and the ``orphaning'' mechanism
in case of nonresponsive authors.  Some articles are world-writeable,
as in the wiki model. & Automated tools for spam and vandalism detection
combined with a system of editorial oversight, in which Jimmy Wales
has last say. & In addition to bug reports and feature requests handled
through the issue tracker, modules can make use of an automated patch
testing system.\\
%\multicolumn{1}{p{.25\textwidth}}{\textbf{Scalability}}
Peer review is distributed.  Links are handled automatically.  Caching
is deployed where relevant; in particular, interlinking features are
kept up to date. & The database and other infrastructure is massively
scaled. There are many bots that help with small tasks. & In theory,
anyone can join.  Earl Miles, NYCCamp 2012 keynote: ``\emph{There are
  no insiders, except Dries; there are no outsiders, only resumes.}''
\\
% \multicolumn{1}{p{.25\textwidth}}{\textbf{Consistency}}
Although automatic links and corrections can help with consistency,
mainly PM relies on standards for proof and expository quality.
&NPOV is the key rule, which works together with templates and other
process tools to maintain community standards about style and
content. & The project issue queues are the place to go when one
module's changes breaks another's. The core of the project has
considerable oversight in these
matters.\\
% \multicolumn{1}{p{.25\textwidth}}{\textbf{Motivation}}
People are solving some of their learning, exposition, and social
needs on the site by writing and reviewing articles and posting in the
forums.& As of 2006, over 50\% of the site had been written by less
than 1\% of the users; these days, paid editing is somewhat
notorious.& Miles continued: ``\emph{To build a resume, find someone
  who needs help, and help them.  Find something that needs doing, do
  it.}'' \\ \hline
\end{tabular}
}
\end{center}
\caption{As typeset ``out of the box''}
\end{table}

\renewcommand{\arraystretch}{1.5}
\begin{table}
\begin{center}
\raisebox{3in}{\small
\begin{tabular}{|p{.25\textwidth}|p{.25\textwidth}|p{.25\textwidth}|}
\hline 
% \multicolumn{1}{p{.25\textwidth}}{\textbf{Relevance}}
\begin{center}(PM)
\end{center}
\nohyphens{
Ultimately relevance depends on peer review, and irrelevant content
may be deleted.  Mechanisms to ensure that relevant content
\emph{will} be added could be improved. } &
\begin{center}(WP)
\end{center}
\nohyphens{ People contribute articles about what
they're interested in; apart from this, rules like WP:WEIGHT come into
play.}& 
\begin{center}(DO)
\end{center}
\nohyphens{
Anyone can upload projects (for ``full projects'', one time approval
is needed), but getting changes into the core requires considerably
more vetting.} \\
%\multicolumn{1}{p{.25\textwidth}}{\textbf{Quality}}
\nohyphens{Quality control is handled through corrections and the ``orphaning'' mechanism
in case of nonresponsive authors.  Some articles are world-writeable,
as in the wiki model.} &\nohyphens{ Automated tools for spam and vandalism detection
combined with a system of editorial oversight, in which Jimmy Wales
has last say.} &\nohyphens{ In addition to bug reports and feature requests handled
through the issue tracker, modules can make use of an automated patch
testing system.}\\
%\multicolumn{1}{p{.25\textwidth}}{\textbf{Scalability}}
\nohyphens{Peer review is distributed.  Links are handled automatically.  Caching
is deployed where relevant; in particular, interlinking features are
kept up to date. }& \nohyphens{The database and other infrastructure is massively
scaled. There are many bots that help with small tasks. }& \nohyphens{In theory,
anyone can join.  Earl Miles, NYCCamp 2012 keynote: ``\emph{There are
  no insiders, except Dries; there are no outsiders, only resumes.}''}
\\
% \multicolumn{1}{p{.25\textwidth}}{\textbf{Consistency}}
\nohyphens{Although automatic links and corrections can help with consistency,
mainly PM relies on standards for proof and expository quality.}
&\nohyphens{NPOV is the key rule, which works together with templates and other
process tools to maintain community standards about style and
content.} & \nohyphens{The project issue queues are the place to go when one
module's changes breaks another's. The core of the project has
considerable oversight in these
matters.}\\
% \multicolumn{1}{p{.25\textwidth}}{\textbf{Motivation}}
\nohyphens{People are solving some of their learning, exposition, and social
needs on the site by writing and reviewing articles and posting in the
forums.}& \nohyphens{As of 2006, over 50\% of the site had been written by less
than 1\% of the users; these days, paid editing is somewhat
notorious.}& \nohyphens{Miles continued: ``\emph{To build a resume, find someone
  who needs help, and help them.  Find something that needs doing, do
  it.}}'' \\ \hline
\end{tabular}
}
\end{center}
\caption{Attempt to control hypenation by using $\backslash$nohyphens\{...\}}
\end{table}

\end{document}