Even the hardest cock is a little more flexible than most dildos. This makes it safer and more comfortable, even if it's stretching you.
A 
Real Cock can feel where it's going and make sure it hits the right spots; also if something goes very wrong a guy can feel it on his cock, whereas a woman with a 
dildo might not notice anything until she pulls it out and sees blood (as happens in a really cheeeeeezy movie called 
Myra Breckenridge).  Bleeding is Not Good unless you're into the VERY heavy black-bandanna stuff.
It's warm and stays warm. 
It gets harder when it's getting close.
Real balls pull up when it's getting close.
It throbs and squirts (which you can sometimes feel even if there's a condom).
The person on the other end of a 
Real Cock is being stimulated at the same time you are, and usually to a similar degree, whereas a woman with a strapon...well, I don't know how that works, actually, but I'm more than sure it's 
different.
I'm sure others can think of more, but that's enough for now.