The ability to get your code out faster than ever! With such an amazing community of people behind it I can't think about anything that would be better in terms of features or support from this platform as compared with others like Xilinx/Altera etc.
If you are looking at getting into FPGA design then definitely look no further because there's not much choice when considering price point but also performance (compared against other platforms). We have been able to build our own SoC designs very quickly using their SDKs which has allowed us more time spent working directly onto hardware rather than software simulations - great benefit!!