Picture yourself as some sort of Creator God, designing humans. What features from other living things would you give us to make us work better than we do now?
Personally, I’d love to have a dolphin’s ability to sleep with half of my brain at a time, so I can make use of all hours of the day instead of spending a third of it unconscious.
On the other hand, they don’t seem to have made many strides in the “Best Original Screenplay” category at the Academy Awards, so they don’t seem to be putting that extra time to good use.
My personal pet peeve is running the optic nerve right through the retina and then putting it on backwards. I’ve seen some pretty poorly thought out designs, but that one is just ridiculous, especially when the cephalopod got it right.
Stranger
Two things. Our knees have not kept up with evolution and aren’t adequate for walking upright. That, and our humeral nerves have no protection. Two of the worst design flaws I can think of.
I’d also change fat processing so that we can eat however much we want of what we want without getting obese. That’s a more selfish feature for me, though. 
It’d be cool if we were constantly regenerating teeth like sharks, instead of just once a lifetime.
I would not add whatever is in the brain/body that allows for addiction. Seriously, does that bit help anyone?
Because of my job, it would be nice to have a bigger brain, and one more suited to multi-tasking.
Because I’m a parent, it would be nice to have those eyes in the back of my head.
I’d also like to be immune to nicotine withdrawal . . . that, or to be immune to negative health effects of smoking.
I’d like to be able to process potato chips and Twinkies as if they were raw vegetables. And I’d burn the most calories by holding down the couch.
I agree with the suggestions proffered so far.
I would add the following:
Separate channels for ingestion and respiration. This would eliminate the possibility of inhaling food. It would also allow me to read to my dear bride whilst ingesting comestibles.
Separate organs for urination and sexual congress. The reasons for this should be obvious.
Retractable testicles. See comment for item 2.
I’d like to make my liver immune to the negative effects of alcohol and my lungs, as mentioned above, impervious to the negative effects of smoke.
I’d like my reproductive organs to be under my direct, conscious control.
There’s no functional aspect to the pain of menstrual cramps, so I’d get rid of those tootsweet.
A prehensile tail would be kinda nice.
Improved liver regeneration.
In addition to knees, as Agent Foxtrot mentioned, our spines never completely adapted to upright walking. I’d fix that, and then I’d give women the ability to dislocate their hips, like snake jaws, when giving birth. Takes care of that problem!
I’d also fix the connections between the brain and the stomach so that the sensations of hunger and satiety were linked to caloric intake and output. A tablespoon of butter would be as filling as two apples!
Oh, and I’d do some brain-tweaking to make us less prone to aggression and violence. We really don’t have much need for it any more.
I just thought of some more stuff. I want the eyes of a hawk, strength of a bear, ears of a wolf, and speed of a puma. I pretty much want to be Brave Starr, without having to live on New Texas.
Improved EVERYTHING regeneration! Major brain injury? Don’t worry, it’ll grow back. Lose an arm? Don’t worry, it’ll grow back. Wanna circumcise your kid? Uh, sorry, you’re out of luck.
Ah yes, those would all be good. I suppose I thought first about which organ of mine is most likely to incur serious damage.
More breasts.
Seriously! Women should have womens breasts but more of them like a cat! six boobs would be awesome!
Alternatively, fix that stupid part of my brain that has a breast fixation!!
The elbow would be a universal joint, to make it easier to wash one’s back.
The five toes would be fused together into one single toe-unit.
I second the idea of retractable testicles.
A third arm would improve things at cocktail parties- one to hold the drink, one to hold the plate of canapes, and one to eat/gesture/shake hands with.
Internal testicles, damn it.
A touch of immortality wouldn’t go amiss.
Yeah, until you try unhooking that bra!
I denuded a girl wearing a basque in less than four seconds! am da man!! 