Besides supporting new hardware optimizations, potential Unified Parallel C support, a conversion to C++, and many other changes, GCC 4.8 improves its support for the C++11 ISO standard.
G++ in GCC 4.8 now supports the C++11 thread_local keyword. "G++ now implements the C++11 thread_local keyword; this differs from the GNU __thread keyword primarily in that it allows dynamic initialization and destruction semantics. Unfortunately, this support requires a run-time penalty for references to non-function-local thread_local variables even if they don't need dynamic initialization, so users may want to continue to use __thread for TLS variables with static initialization semantics. OpenMP threadprivate variables now also support dynamic initialization and destruction by the same mechanism."
Another C++11 feature for this GCC release due out in 2013 is support for the attribute syntax, alignment specifier, and inheriting constructors.
The early C++1y support is exposed via setting the -std=c++1y switch but right now the only proposed feature that is implemented is support for return type deduction in normal features. This next C++ standard isn't expected to be settled until 2017.