While #246 added basic support for the array environment, that support is not complete yet. It would be nice to get it (more) complete one day, so let's keep track of what's still missing, since filing a bug for each of these things feels inappropriate just now.

In the format description:

• | – vertical lines between columns (Add horizontal and vertical line support in environments #267)
• || – i.e. repeated vertical lines
• *{7}{cc} – repetition in format description
• @{=} – i.e. custom coumn separators
• \extracolsep for use in @{…}
• p{2em} – inserts \parbox of given width

In the body:

• \hline – horizontal lines (Add horizontal and vertical line support in environments #267)
• \hline\hline – repeated horizontal lines
• \cline[1-3] – lines spanning some of the columns
• \multicolumn{1}{c}{Foo} – to change the formatting of a single cell
• \multicolumn{2}{c}{Foo} – content spanning multiple columns

In individual cells:

• \hfill at the ends to change the alignment of a single cell
• \hfill within the content to split it into a left-aligned and a right-aligned part
• \vline – vertical line as high as the cell

Format descriptions from the array package:

• <{foo} and >{bar} – add content to all cells
• m{1em} and b{3em} – like p but with different vertical alignment
• !{=} – like @ but keep the space

I'm not sure we'll ever want to support all of this.