A style prevents me from setting \belowcaptionskip, how do I change that?

Further to @egreg's answer, the center environment adds the additional vertical space around your tabular, causing the visually large gap, hence the suggestion to use \centering. Also, the llncs document class executes

\renewenvironment{table}
  {\setlength\abovecaptionskip{0\p@}%
   \setlength\belowcaptionskip{10\p@}%
   \@float{table}}
  {\end@float}

which removes \abovecaptionskip and fixes \belowcaptionskip to 10pt at every use of table (actually, it does the same for table*). If you wish to remove this restriction altogether, and perhaps have more freedom over your caption spacing, you have the following choices:

• manual assignment of these lengths inside the environment (table or table*), immediately after \begin{...} or before \caption[..]{...}; or

• automatic removal (or modification) these the length restriction by means of the etoolbox package. For this use

  \usepackage{etoolbox}% http://ctan.org/pkg/etoolbox
  ...
  \makeatletter
  \patchcmd{\table}{0\p@}{5\p@}{}{}% \abovecaptionskip @ 0pt -> 5pt
  \patchcmd{\table}{10\p@}{5\p@}{}{}% \belowcaptionskip @ 10pt -> 5pt
  \makeatother
  

  which patches the beginning of the table environment. The command \patchcmd has the format \patchcmd{<command>}{<search>}{<replace>}{<success>}{<failure>} that searches for <search> in <command> and replaces it with <replace>. Additionally, it executes <success> if this patch was successful, and <failure> if not. Since the patch works, there's need to bother with <success> or <failure>, hence they are both left empty {}. The modification to 5pt was arbitrary and can be adjusted to your liking. This is, of course, dependent on whether article submissions allow such modifications in the first place.

There are two aspects of the problem.

First the main one: don't use the center environment for centering the table, but rather \centering.

\begin{table}[!htb]
\centering
\caption{Examples for requirement the target users preferred}
\label{tab:features_liked}
\begin{tabular}{|p{90mm} | p{30mm}|}
...
\end{tabular}
\end{table}

Second. The llncs class sets the \belowcaptionskip when doing \begin{table}, so a previous setting in the preamble will be ignored. However, this setting is to 10pt, which is reasonable.

So, don't use center.