I'm no expert in the field, but I'd like to bring up neuroplasticity. Our brains are constantly rewiring themselves as they process input, and they gradually adjust to change. My point is that I believe any enhancement could come to feel natural (although some would certainly have a higher learning curve).

Other thoughts:

• Ever read Uglies, Pretties, and Specials by Scott Westerfield? It's set in a utopia/dystopia where massive plastic surgery is the norm - at 16, everyone chooses what they will look like (going from "Ugly" to "Pretty") and similar changes occur at middle age, and so on. One of the points made is that there will always be something to envy - if it stops being looks it'll become something else.

• I'd take some kind of physical enhancement that removes most bodily needs - sleeping, bathroom, eating, etc. - although this is a symptom of the more general "anything that gives me more free time is good" heuristic.

• I can imagine some kind of gene sequencing becoming a regular medical practice - stripping people of bad genes, or enhancing good ones.