New in C9X (in no particular order)

• more precise aliasing rules via effective type
• restricted pointers
• variable-length arrays
• flexible array members
• static and type qualifiers in parameter array declarators
• complex (and imaginary) support in <complex.h>
• type-generic math macros in <tgmath.h>
• the long long int type and library functions
• increased minimum translation limits
• additional floating-point characteristics in <float.h>
• remove implicit int
• reliable integer division
• universal character names (\u and \U)
• extended identifiers
• hexadecimal floating-point constants and %a and %A printf/scanf conversion specifiers
• compound literals
• designated initializers
• // comments
• extended integer types and library functions in <inttypes.h> and <stdint.h>
• remove implicit function declaration
• preprocessor arithmetic done in intmax_t/uintmax_t
• mixed declarations and code
• new block scopes for selection and iteration statements
• integer constant type rules
• integer promotion rules
• macros with a variable number of arguments
• the vscanf family of functions in <stdio.h> and <wchar.h>
• additional math library functions in <math.h>
• floating-point environment access in <fenv.h>
• IEC 60559 (also know as IEC 559 or IEEE arithmetic) support
• trailing comma allowed in enum declaration
• %lf conversion specifier allowed in printf
• inline functions
• the snprintf family of functions in <stdio.h>
• boolean type in <stdbool.h>
• idempotent type qualifiers
• empty macro arguments
• new struct type compatibility rules (tag compatibility)
• additional predefined macro names
• _Prama preprocessing operator
• standard pragmas
• __func__ predefined identifier
• va_copy macro
• additional strftime conversion specifiers
• LIA compatibility annex
• deprecate ungetc at the beginning of a binary file
• remove deprecation of aliased array parameters
• conversion of array to pointer not limited to lvalues
• relaxed constraints on aggregate and union initialization
• relaxed restrictions on portable header names
• return without expression not permitted in function that returns a value (and vice versa)

New in C93 (AM1)

• restricted character set support via digraphs and <iso646.h>
• wide character library support in <wchar.h> and <wctype.h>