With the new activity diagram syntax, the default direction of the 'if' and 'else' branches are left and right, respectively. As a consequence, diagrams tend to 'expand to the left' a lot.
A request for customizable directions exists, and would be very nice if it could be implemented in any case, but is so far not answered.
IMHO diagrams would already become more readable if the default direction would become 'down' instead for the 'if'-branch when there is no 'else' or the 'else' is empty (i.e. only present to put a label on the 'else'-branch, but no operations in it) or with only a 'stop'. Alternatively, if the 'if'-branch is empty or contains only a 'stop' (as in some branches of the example given by http://plantuml.sourceforge.net/qa/?qa=2961/activity-beta-drawing-improvements where the reversal is clearly used to improve the layout also) and the 'else'-branch is not, the 'else'-branch should go down instead.
This behaviour (the first part of it) is already default if there is an 'elseif' following the 'if', and for each 'elseif' as well, so should maybe not be too complicated to implement?
However, I understand that this is a matter of taste, so maybe the default behaviour should be configurable with a 'skinparam' option for the diagram?