Howto achieve capitalized description in glossary table?

You could use the description option of glossaries which is probably meant to add a different description to the list of acronyms than the long form of the acronym. But this means it also works if description and long form only differ in the first letter. This is obviously not as handy as David's solution since you have to type each form twice...

\documentclass[12pt]{article}
\usepackage[acronym,description]{glossaries}
\makeglossaries

\newacronym[description=This is a test]{test}{Test}{this is a test}

\newglossaryentry{web}{
  name={World Wide Web},
  first={World Wide Web (WWW)},
  text={WWW},
  description={A distributed system of linked hypertext documents}
}
\begin{document}

Here comes \gls{test}

Here comes \gls{web}

\printglossary[type=\acronymtype]

\printglossary
\end{document}

Something like this works for \printglossaries:

\let\firstchar\lowercase
\let\oldprintglossaries\printglossaries
\def\printglossaries{\let\firstchar\uppercase\oldprintglossaries}

\newacronym{test}{Test}{\firstchar{t}his is a test}

Working example

\documentclass[a4paper]{article}
%
\setlength{\parindent}{0pt}
%
\usepackage{xcolor}
\usepackage{hyperref}% Must now be the second-last package that is loaded
\usepackage[acronym]{glossaries}% Must be last package, loaded after hyperref
%
\newglossaryentry{htdoc}{name={hypertext document},description={Text file with references to other content via hyperlinks}}
\newglossaryentry{web}{name={World Wide Web},first={World Wide Web (WWW)}, text={WWW},description={A distributed system of linked hypertext documents}}
%
\let\firstchar\lowercase
\let\oldprintglossaries\printglossaries
\def\printglossaries{\let\firstchar\uppercase\oldprintglossaries}

\newacronym{test}{Test}{\firstchar{L}owercase is what I want}
\makeglossaries

\begin{document}

% Test of glossary entries
% 1. Capitalization
Here is the acronym in the text \gls{test}
% HTML should not appear in acronym list because not cited and not added
%
\printglossaries
%
\end{document}

NoIdx version

If you use \printnoidxglossaries instead, use these redefinition instead:

\let\firstchar\lowercase
\let\oldprintnoidxglossaries\printnoidxglossaries
\def\printnoidxglossaries{\let\firstchar\uppercase\oldprintnoidxglossaries}