Ctrl-C does not always work to interrupt a stuck Vim process. By making Cmd-. send SIGINT it is more likely to succeed where Ctrl-C has failed. (E.g. Ctrl-C may fail if a DO message is dropped, or if the Vim process is stuck in a loop and isn't checking for new input.)
Previously, all output was sent to the frontend at once so only the last few rows were visible. This was a bit disconcerting when a command took a bit of time because there was no feedback that anything was happening. The downside to this patch is that it takes a bit longer for a command to finish since the frontend has to do more drawing.
Ignoring SIGCHLD caused problems with automatic updating (Sparkle) since it uses popen() (and hence implicitly uses wait4()) to unpack archives. Now that SIGCHLD is no longer ignored we have to reap child processes after exiting a Vim process as well as when MacVim is about to terminate.
When enabled the default font is loaded from within the app bundle each time MacVim is started (the default setting). Startup times are shorter if this is disabled. Note that if the default font has already been installed on the system then this may as well be disabled.
When adding input programmatically the 'silent' flag is automatically set. Hopefully this will mean the end of the annoying "Press ENTER..." prompt. This also means that it should no longer be necessary to manually try to make commands silent (e.g. ":redr|f" should not be needed).