Here are a few:

1. Git push should default to --force-with-lease --force-if-includes.

2. push.autoSetupRemote should be enabled by default.

3. The default conflict style should be zdiff3.

4. diff.submodule should be 'log' by default (gives much nicer submodule diffs).

5. Submodule updates / clones should be recursive by default. (There is a setting for this but I can't remember it.)