Some trivial inconsistencies in Phobos functions.
Some of them are quite annoying.

std.utf:
string toUTF8(string s);  -> string toUTF8(const(char)[] s);

std.string:
const(char)* toStringz(const(char)[] s); 
      -> immutable(char)* toStringz(const(char)[] s);

(Returning a const is always suspicious).

std.stream:
void write(char[] s); -> void write(const(char)[] s); 
size_t printf(char[] format,...);  -> size_t printf(const(char)[] format,...);

std.encoding:
abstract const string toString(); -> abstract string toString();