--keep-going is a very useful feature, I and (I hope) many others use it often when upgrading the system. I think it would save people a lot of typing if it could be set once and for all in make.conf in FEATURES. Reproducible: Always
You can already set it in /etc/make.conf: EMERGE_DEFAULT_OPTS="--keep-going"
Yeah, there's no point in adding this to FEATURES since that's what EMERGE_DEFAULT_OPTS is for. However, we can add support for --keep-going=n in case you want to disable it on the commandline.
ups. Sorry, didn't know about this EMERGE_DEFAULT_OPTS. Thanks ;)