A fancy and beautiful type of enumerate

Here is a very naive and brute force proposal.

\documentclass{article}
\usepackage{tikz}
\newcommand{\SebastianoItem}[1]{\tikz[baseline=(SebastianoItem.base),remember
picture]{%
\node[fill=red!20,inner sep=4pt,font=\sffamily] (SebastianoItem){#1};}}
\newcommand{\SebastianoHighlight}{\tikz[overlay,remember picture]{%
\fill[red!20] ([yshift=4pt,xshift=-\pgflinewidth]SebastianoItem.east) -- ++(4pt,-4pt)
-- ++(-4pt,-4pt) -- cycle;
}}
\begin{document}
\renewcommand{\labelenumi}{\SebastianoItem{\arabic{enumi}}}
Some basic rules.
  \begin{enumerate}
    \item Never put pineapple on a pizza!
    \item Never!!!! \SebastianoHighlight
    \item Trust me!
  \end{enumerate}
\end{document}

ADDENDUM: Brackets and colors. (Of course, if you have longer lists, you need to either extend the color list, or implement some mod condition.)

\documentclass{article}
\usepackage[x11names]{xcolor}
\usepackage{tikz}
% from https://tex.stackexchange.com/a/167024/121799
\newcommand{\ClaudioList}{red,DarkOrange1,Goldenrod1,Green3,blue!50!cyan,DarkOrchid2}
\newcommand{\SebastianoItem}[1]{\foreach \X[count=\Y] in \ClaudioList
{\ifnum\Y=#1\relax
\xdef\SebastianoColor{\X}
\fi
}
\tikz[baseline=(SebastianoItem.base),remember
picture]{%
\node[fill=\SebastianoColor,inner sep=4pt,font=\sffamily,fill opacity=0.5] (SebastianoItem){#1)};}
}
\newcommand{\SebastianoHighlight}{\tikz[overlay,remember picture]{%
\fill[\SebastianoColor,fill opacity=0.5] ([yshift=4pt,xshift=-\pgflinewidth]SebastianoItem.east) -- ++(4pt,-4pt)
-- ++(-4pt,-4pt) -- cycle;
}}
\begin{document}
\renewcommand{\labelenumi}{\SebastianoItem{\arabic{enumi}}}
Some general advices.
  \begin{enumerate}
    \item No fast food.
    \item Don't drink to much alcohol.
    \item No pineapple on pizza. \SebastianoHighlight
    \item Don't use onions in food.
  \end{enumerate}
\end{document}

Without tikz.

Here I introduce the bangenumerate environment, in which emphasized items are invoked with \item! instead of \item. The optional argument of \item is still supported.

(Note: the syntax \item[]! has no meaning, as the ! directive is overridden with the optional label. )

Color can be changed mid stream with an invocation of \colorlet{bangcolor}{...} between \items.

EDITED to \smash the label, so that larger values of \fboxsep can be employed without affecting line spacing.

\documentclass{article}
\usepackage{xcolor,enumitem,amssymb}
\let\svitem\item
\let\thebang\relax
\colorlet{bangcolor}{red!50}
\newenvironment{bangenumerate}
{%
  \fboxsep=3pt\relax%
  \renewcommand\item[2][\relax]{%
    \ifx!##2%
      \def\thebang{\makebox[0pt][l]{\kern-1pt\textcolor{bangcolor}{%
        \raisebox{1pt}{$\scriptstyle\blacktriangleright$}}}}%
      \def\next{}%
    \else%
      \def\thebang{}%
      \def\next{##2}%
    \fi%
    \ifx\relax##1\relax\svitem\else\svitem[##1]\fi\next%
  }%
  \begin{enumerate}[label={%
    \smash{\colorbox{bangcolor}{\bfseries\sffamily\theenumi)}\thebang}}]%
}{\end{enumerate}}
\begin{document}
\begin{bangenumerate}
\item My macro for personalized \textbf{enumerate}. 1) or 2) or 3) \ldots has been adding without 
\item automatic enumeration.
\item! This is that I have tried, quickly, only this morning. 
\item[OTHER] It is very nice but not as the figure with the little arrow (for important enumerate).
\item Should be normal without a bang
\colorlet{bangcolor}{blue!30}
\item! Color change with bang
\end{bangenumerate}
\end{document}