If you really wanted to swap the hard drive, nothing is stopping you except for the fact that you may be voiding your warranty and your technical skills.
I'm in the camp that believes if you want a blu-ray player, it belongs under your TV. Why is there a DVD drive in Macs? To install software.
Graphics card? Well, you know what you are buying and that it will never be upgradable. Don't like it? Don't buy it.