The only thing I miss, which I think would be a *very* useful (optional) view mode, is the following behaviour when comparing 2 files side by side:
To be able to switch to 'line' or 'list' mode where you can select a certain line, and view the contents of that line in more detail at the bottom of the window, with the line contents of the left panel showing directly above the contents of the right panel. In this way you can a) see much more text because it uses the whole screen width, and b) it's easier to spot the differences when they are in line with each other.
An important side effect of viewing files in this line-by-line (list) mode is that, when viewing 'Differences only', you can select an interesting line, then switch back to 'Show All' mode and the window is automatically scrolled to show that same selected line in context. In this way the 3 views - Show All, Differences only and Differences with context - all stay in sync with each other.
That said, I find that DiffMerge, in its present form, is a joy to use. Kudos to the developers!