Revit 2015 introduces some new revision tool functionality. I’ve outlined the 3 biggest changes from 2014, along with some additional information:
Draw Tools
Instead of a generic cloud tool that required a lot of clicks, the standard draw tools are now available. Rectangle is set as the default for revision cloud sketches:

Delete Revisions
Revisions can now be deleted, which will remove the revision from the project and all associated clouds \ tags. You cannot delete an issued revision:

Arc Length
On a project-wide basis you can control the maximum cloud segment size (the “double bubble”). For example, if Arc Length is set to 2”, and you are drawing a revision cloud on a sheet directly, after the segment passes 2” a second segment would begin:

Some quick additional tidbits to keep in mind:
Sheet Scaling
With Revit 2015 revision clouds are treated similar to annotation and scale according to the view scale. For example, if you were to draw the clouds directly on a sheet the exact arc length value would utilized.
Arc Length
Revit has always used a special “double bubble” cloud segment. The Arc Length value refers to the maximum allowed sheet space size per double arc segment (see above Arc Length image).
Upgrade Arc Length
By default the Arc Length is set to 3/4” for new 2015 projects. If your previous-release project contains revision clouds, when you upgrade to Revit 2015 the Arc Length is set to 3 1/2”. For most cases this will prevent revision cloud geometry changes on upgrade. Then, on a project basis, you can set it to the desired size.
Small Segments
You can always create segments smaller than the specified Arc Length.