That’s so cool. Do you remember the exact species? 
Definitely a “rete mirabile”. It would make possible maintaining rectal temperatures of at least 46.5 degrees C for as long as 6 hours with no apparent ill effects, though that isn’t exactly what I’d want it for…
I think many desert animals have a similar ability, since they can’t afford to waste any water. Desert tortoises come to mind.
Yeah… but I can’t help thinking some folks might drown in the rain. 
When I got a pet hamster as a kid, I first researched the little critters and found out that hamsters only excrete solid waste. They’re adapted to the desert, so their bodies use water very efficiently. That would be pretty cool.
Extendable/vibrating penis.
What existing animal has a vibrating penis?
I wouldn’t mind a better tumor suppressor gene.
How about… lenses that retain their flexibility as people get into their forties? Then we wouldn’t have to buy reading glasses.
Though if you’re going to redesign the eye, I’ll go with the nerves on the proper side of the retina, behind the sensor cells (as mentioned above), plus a wider reception bandwidth so that we can see into the infrared and ultraviolet.
The ability to safely drink sea water.
Appetite needs to be more closely linked to physical needs. Appetite for fatty foods should go away when there’s enough fat reserve in the body. Same for salt. And deficiency in a specific nutrient should trigger a craving for food containing that nutrient.
Actually yes. The way addiction is believed to work is by hijacking the parts of the brain responsible for the feelings of pleasure and reward. If we didn’t have that circuitry anymore, life would be pretty dull.
Thanks for the info.
I agree. I wonder, though, if we only had solid waste if we’d be more prone to polyps.
Regeneration
Hibernation
Breath water and air
Bioluminescence, of the fingers, perhaps, so we can find things in the dark easier.
Eliminate itching entirely.
Replace pain with a text message in the corner of your visual field that tells you exactly what’s wrong with what part of your body. Something like “Left Ankle Sprained – Healing Progress 13%” or “4th Vertebra Herniated Disc – See Doctor Immediately”.
Whales penises are prehensile: http://www.youtube.com/watch?v=6xJFR5P7BdA
There’s a better clip on the series Blue Planet, but I can’t find it on youtube.
I bet hummingbird penises vibrate.
Asexual reproduction
Seriously, if we had this, imagine the productivity of our species. No longer would our lives be consumed with seeking and finding mates. This one thing would massively reorganize every aspect of society from social aspect, religion, politics, family, etc.