I see two features: (1) that we can intercept senses, so a car, bike or horse (or spear, axe or hammer), is neurologically an extension of ourselves; (2). (I think) abstract reasoning has an identical neurological basis to that used by rats, crows and dolphins to solve puzzles, except over abstract concepts instead of over concrete instances.
> a possible test for whether a cognitive enhancement is likely to have downsides, is the ease with which it could have occurred as a natural mutation
Our environment is different today, and traits that would have often killed their possessors 100,000's of years ago could be beneficial today. I'm thinking things like Asperger Syndrome, even Autism (which might have factors that are advantageous in a different combination, like Sickle Cell anemia - one grant malaria immunity; two kills you).
Even the earliest agriculture could have profoundly distorted the fitness function, so we may been evolving very rapidly over our recent genealogical history (rather than stagnating as is often claimed).
Better short-term memory would increase effective intelligence (in my personal experience, anyway), and it seems that evolution could improve this without harm...? Or maybe that's just von neuman bias... it could be that short-term memory is intimately integrated with intelligence - eg. each chunk in working memory is connected to everything else in long-term memory, allowing you to recognize similarities etc. You couldn't afford many buses like that.