Some useful information here:
http://mathematica.stackexchange.com/questions/54607/mathematica-10-dataset-doesnt-format-more-than-4-columns
Taliesin Beynon, one of the commenters/responders in that thread is the lead developer on much of the new functionality surrounding Dataset, associations, and related things in Mathematica 10.
I think that, with regard to the default output formatting of Dataset, Taliesin is saying that it is not currently user-adjustable but that it will evolve in the future. I looked for system settings that determine this (similar to ones that determine how output formats are truncated for large expressions, for example) and did not immediately find any.
As Pieter suggests, for Datasets that involve strictly rectangular data, one could create a custom visualization of what's in the dataset starting with something fo the form
Grid[Normal[Normal[dataset]] /. {(_ -> z_) :> z}]
and building on this to tweak the appearance and, for example, to specify how many rows & columns to show by default.
