Automatic memory management, instantaneous compilation, no portability issues, (easier) polymorphism. Maybe weeks -> days was an overexaggeration when it comes to C -> Java but I definitely experienced a big productivity gain, and it wasn't because of a lack of C experience.

From a quick glance at the docs glib looks good but it is still too low-level for normal app development (for example, the size of a hashing table created by hcreate() is fixed; so you have to create & copy & destroy a hash table if you want to add more elements than it can hold at a time) and it should be, that's the whole point.

C++ is a whole other story, sure.