Theoratically it is being said that we cannot call main
recursively(main caling main) in c++, but practically it can
be done.

whereas in c, we can call main in main.

Pointers we can do it in  c but not in c++

we can use printf in c but not in c++

there are many concepts that can be used in c++ where c 
cannot support those concepts.
The following are some of those type of concepts:-
 1.function overloading
 2.operator overloading
 3.templates
 4.polymorphism
 5.inheritence
 6.data encapsulation
 7.function over-riding
 8.virtual functions
 9.constructors & destructors
 10. exceptional handling.

nothing

every C program is execute in C++.
From the above reason we can understand that every thing in
C we have in c++ also.
so Answer is NOTHING

try this one
int *x=malloc(10);
it works in c but not in c++ reason is we will have to 
explicitly cast in c++ to convert a void* to another type 
to work.

C++ retained all features of C programming in addition of 
object oriented programming that use OBJECT and a CLASS so 
there is nothing that happen in C but not in C++;

One important difference between c and c++ is that c 
compilers are loosely coupled. So it does not perform type 
checking that much strictly and can allow some conversions 
which otherwise most c++ compilers would not allow.