You can return to V10 dataset styling using:

    Dataset`$UseNewDatasetFormatting = False;

As we shall see below, this will restore the grid-like visualization for some datasets similar to the one exhibited in the question.  However, for the question's dataset V10 also does not expand the assocation lists into grid columns:

[![v10 vs v11 screenshot][1]][1]

But... there are definitely some significant differences between V10 and V11 rendering.  Consider the following datasets:

    integers[] :=     <| "a" -> { 1,   2 }, "b" -> { 4,   5 }, "c" -> { 7,   8 } |> // Dataset
    strings[] :=      <| "a" -> {"1", "2"}, "b" -> {"4", "5"}, "c" -> {"7", "8"} |> // Dataset
    symbols[] :=      <| "a" -> {a1,  a2 }, "b" -> {a4,  a5 }, "c" -> {a7,  a8 } |> // Dataset
    mixedColumns[] := <| "a" -> { 1,  a2 }, "b" -> { 4,  a5 }, "c" -> { 7,  a8 } |> // Dataset
    mixed[] :=        <| "a" -> { 1,  a2 }, "b" -> {"4", a5  }, "c" -> { 7, a8 } |> // Dataset
    
    $datasets = {integers, strings, symbols, mixedColumns, mixed};

The following table shows the differences between the V10 and V11 rendering:

    oldAndNewBoxes[n_] :=
      { n
      , Block[{Dataset`$UseNewDatasetFormatting = False}, MakeBoxes[#]&[n[]] // RawBoxes]
      , MakeBoxes[#]&[n[]] // RawBoxes
      , Dataset`GetType[n[]]
      }
      
    $datasets // Query[Prepend[{"", "V10", "V11", "Data Type"}] /* Grid, oldAndNewBoxes]

[![v10/v11 comparison table][2]][2]

We can see that V10 would render the association values as grid columns for a wider range of data types than the new V11 visualization does.  This might be a bug worth reporting to WRI.


  [1]: https://i.sstatic.net/Gb1R2.png
  [2]: https://i.sstatic.net/5Vrsu.png