How to make beamer overlays with Tikz node

I used to do this according to Andrew's solution until I read his note #2, and it reminded me that PGF's keys can do pretty much anything. The key (excuse the pun) is to create a key that processes other keys conditional on the slide number:

\tikzset{onslide/.code args={<#1>#2}{%
  \only<#1>{\pgfkeysalso{#2}} 
}}

Using \pgfkeysalso doesn't reset the current key path, whereas \pgfkeys or \tikzset would. The .code args key handler means that

onslide=<overlay specification>{keys}

causes the the following code to be expanded:

\only<overlay specification>{\pgfkeysalso{keys}}

Then you can use the key onslide=<overlay specification>{keys} to set keys only on specific slides. The slightly inelegant twist is that if your overlay specification contains commas the entire pair of overlay spec and keys has to be embraced.

Here is a complete example:

\documentclass{beamer}
\usepackage{tikz}
\usetikzlibrary{positioning}

\tikzset{onslide/.code args={<#1>#2}{%
  \only<#1>{\pgfkeysalso{#2}} % \pgfkeysalso doesn't change the path
}}
\tikzset{temporal/.code args={<#1>#2#3#4}{%
  \temporal<#1>{\pgfkeysalso{#2}}{\pgfkeysalso{#3}}{\pgfkeysalso{#4}} % \pgfkeysalso doesn't change the path
}}

\tikzstyle{highlight}=[red,ultra thick]


\begin{document}
\begin{frame}
 \begin{figure}[h]
  \begin{centering}
  \begin{tikzpicture}[
    system/.style={draw,rectangle,rounded corners=3,minimum width=2cm,text width=1.8cm,text centered},
    node distance=2cm
  ]
    \node [system,onslide=<3->{highlight},anchor=center] (fe) {Feature Extraction};
    \node [system,onslide={<2,4>{green}}] (am) [right=of fe.center] {Acoustic Model};
    \node [system,temporal=<3>{blue}{highlight}{green}] (lm) [right=of am.center] {Language Model};
    \node [system] (d) [below=of lm.center] {Decoder};

    \draw[->] (fe) |- (am);
    \draw[->] (am) |- (d);
    \draw[->] (lm) -- (d.north);

  \end{tikzpicture}
  \end{centering}
  \end{figure}
\end{frame}
\end{document}

You will be disappointed about the jiggling of your picture when the line thickness keys are applied. You might have to avoid relative positioning to make that go away.

For more on key handlers like .code args, see the pgfkeys section of the TikZ-PGF manual.

There are a few ways to do this, exactly which is best will depend on what you want the highlighting effect to be. One way is to have separate nodes with the two styles and then show one on one slide and one on the other. Another way is to define a style that is wrapped in an \only. Actually, using beamer's reimplementation of \newcommand, one can make \tikzset overlay-aware. Here's a couple of possibilities:

\documentclass{beamer}
\usepackage{tikz}
\usetikzlibrary{positioning}

\newcommand<>{\btikzset}[2]{\alt#3{\tikzset{#1}}{\tikzset{#2}}}

\tikzset{highlight/.style={red,fill=green,text=blue}}

\begin{document}
\begin{frame}
\begin{figure}[h]
  \begin{centering}
  \begin{tikzpicture}[system/.style={draw,rectangle,rounded corners=3,minimum width=2cm,text width=1.8cm,text centered}]
    \node [system] (fe) {Feature Extraction};
    \node<1> [system] (am) [right=of fe] {Acoustic Model};
    \node<2> [system,highlight] (am) [right=of fe] {Acoustic Model};
    \node [system] (lm) [right=of am] {Language Model};
    \node [system] (d) [below=of lm] {Decoder};

    \draw[->] (fe) |- (am);
    \draw[->] (am) |- (d);
    \draw[->] (lm) -- (d.north);

  \end{tikzpicture}
  \end{centering}
  \end{figure}
\end{frame}
\begin{frame}
\begin{figure}[h]
  \begin{centering}
  \begin{tikzpicture}[system/.style={draw,rectangle,rounded corners=3,minimum width=2cm,text width=1.8cm,text centered}]
  \btikzset<2>{highlighton2/.style={highlight}}{highlighton2/.style={}}

    \node [system] (fe) {Feature Extraction};
    \node [system,highlighton2] (am) [right=of fe] {Acoustic Model};
    \node [system] (lm) [right=of am] {Language Model};
    \node [system] (d) [below=of lm] {Decoder};

    \draw[->] (fe) |- (am);
    \draw[->] (am) |- (d);
    \draw[->] (lm) -- (d.north);

  \end{tikzpicture}
  \end{centering}
  \end{figure}
\end{frame}
\end{document}

Notes:

1. I found that \renewcommand<> didn't work, possibly because I was making \tikzset take two arguments - thus having it work like \alt rather than \only. This was to avoid complaints of unknown keys. An alternative would be to define an empty style, say highlightOn at the start and then have \tikzset map highlightOn to highlight on frame 2 and do nothing on other frames. This would work with \renewcommand<>:

   \renewcommand<>{\tikzset}[1]{\only#2{\beameroriginal{\tikzset}{#1}}}
   
   
   \tikzset{highlight/.style={red,fill=green,text=blue}}
   \tikzset{highlighton2/.style={}}
   

   Then in the frame:

   \tikzset<2>{highlighton2/.style={highlight}}
   

2. Perhaps the truly elegant way would be to define a new key, "highlight on=", using the code handler. But I've only just learnt of the existence of that so I'll leave that solution to someone else.