I was under the impression that "brain space" was unlimited for all practical intents and purposes, and that having more stuff in your brain might actually even make extra learning easier - e.g. I've often heard it said that a person loses fluid intelligence when they age, but this is compensated by them having more knowledge that they can connect new things with. Do you know of studies to the contrary?

What do you outsource that is common and uncommon among people that you know?

A lot of little facts (of the kind that people on LW use Anki decks to memorize). I outsource them to Google.

I barely remember any phone numbers nowadays and that seems to be common.

What should you be avoiding keeping in your brain that you currently are?

Schedules / to-do lists. I really should outsource them to some GTD app, but can't bring myself to use one consistently.

Dunno about that; in my case information is either worth knowing (like 'what poplar tree marks the turn left to Epipactis palustris' - ideally outsourced to a map, but I would just get confused, or 'what kind of porridge to cook tonight given the kid rejected x, y and z' - ideally outsourced to a notebook, but there are too many details to bother with it) or worth losing.

When I am ill, though, I outsource my meds list to the fridge door. (Recipes and shopping lists, too, occasionally.)

I'm not sure whether it makes me a more satisfied/happy person if I out-source lots of things to devices. I agree that it is likely more efficient to delegate lots of memory work and planning habits to devices. But it also takes some of your autonomy away. It of course depends on the specific interaction and probably also on the person (some people may feel it quite natural to delegate tasks to (virtual) persons they trust). But as long as the out-sourced task affects you later in a non-adaptive way (and I judge this to be mostly the case) then this might not feel as natural as one might like.

See also my post about when augmentations feel/are natural.