Wierd you asked this because I sent off for my provisional license today and on it they asked if you'd go on the organ donor register or not. I ticked yes straight away because it's something I've said I'd always do ... once I'm dead, they are no use to me so I see no reason not to. If others are going to benefit then why not ...
I think it should be compulsary, if it's potentially saving people's lives then what's the harm or the big deal in it? You don't need them, you're dead ... don't be selfish.